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The quasi-two-dimensional antiferromagnet Ni$_2$P$_2$S$_6$ belongs to the family of magnetic van der Waals compounds that provides a rich material base for the realization of fundamental models of quantum magnetism in low dimensions. Here, we report high-frequency/high-magnetic field electron spin resonance measurements on single crystals of Ni$_2$P$_2$S$_6$. The results enable one to reliably determine the positive, "easy"-plane type of the single ion anisotropy of the Ni$^{2+}$ ions with the upper limit of its magnitude $\lesssim 0.7$ meV. The resonance response reveals strongly anisotropic spin fluctuations setting in shortly above the N\'eel temperature $T_{\rm N} = 158$ K and extending in the antiferromagnetically ordered state down to low temperatures. There, a low-energy magnon excitation gapped from the ground state by only $\sim 1$ meV was found. This magnon mode may explain unusual low temperature relaxation processes observed in a latest nuclear magnetic resonance experiments and together with the estimate of the single ion anisotropy they should enable setting up a realistic spin model for an accurate description of magnetic properties of Ni$_2$P$_2$S$_6$.

Current sheets (CSs) are preferred sites of magnetic reconnection and energy dissipation in collisionless astrophysical plasmas. Electric currents in them may be carried by both electrons and ions. In our prior theoretical studies of processes associated with the CS formation in turbulent plasmas, for which we utilized fully kinetic and hybrid code simulations with ions considered as particles and electrons - as a massless fluid, we found that electron-dominated CSs may form inside or nearby ion-dominated CSs. Electrons become the main carrier of the electric current and contributors to energy dissipation in electron-dominated CSs. These magneto-plasma structures represent a distinguished type of CSs and should not be mixed up with so-called electron-scale CSs. Our simulations show that such CSs are characterized by the electron-to-ion bulk speed ratio Ue/Ui increases. Theoretical predictions and high-resolution observations from the MMS mission suggest that strong electron-dominated CSs can be seen at ion scales. Therefore, we suggest that applying the Ue/Ui parameter to the solar wind data may allow locating the strongest electron-dominated CSs with an ordinary spacecraft resolution of one-three seconds at least approximately. The results show that, indeed, an impact of electron-dominated CSs on the plasma observed during a period of quiet solar wind conditions at 1 AU may be associated with sharp changes in Ue/Ui. Electron-dominated CSs are found to be localized in the vicinity of ion-dominated CSs identified via changes in the magnetic field and plasma parameters Ue/Ui, displaying the same clustering. We conclude that Ue/Ui may be used as one of key parameters for probing CSs in the solar wind and the role of electrons in them.

We report the anomalous breakdown in the scaling of the microscopic magnetic susceptibility, as measured via the ${}^{31}$P nuclear magnetic resonance (NMR) shift $K$, with the bulk magnetic susceptibility $\chi$ in the paramagnetic state of Mn$_2$P$_2$S$_6$. This anomaly occurs near $T_\mathrm{max} \sim 117$ K the maximum in $\chi(T)$ and is therefore associated with the onset of quasi-two-dimensional (quasi-2D) magnetic correlations. The spin-lattice relaxation rate divided by temperature $(T_1T)^{-1}$ in Mn$_2$P$_2$S$_6$ exhibits broad peak-like behavior as a function of temperature, qualitatively following $\chi$, but displaying no evidence of critical slowing down above the N\'{e}el temperature $T_N$. In the magnetic state of Mn$_2$P$_2$S$_6$, NMR spectra provide good evidence for 60 degree rotation of stacking-fault-induced magnetic domains, as well as observation of the spin-flop transition that onsets at 4 T. The temperature-dependent critical behavior of the internal hyperfine field at the P site in Mn$_2$P$_2$S$_6$ is consistent with previous measurements and the two-dimensional anisotropic Heisenberg model. In a sample of Ni$_2$P$_2$S$_6$, we observe only two magnetically split resonances in the magnetic state, demonstrating that the multiple-peaked NMR spectra previously associated with 60 degree rotation of stacking faults is sample dependent. Finally, we report the observation of a spin-flop-induced splitting of the NMR spectra in Ni$_2$P$_2$S$_6$, with an onset spin-flop field of $H_\mathrm{sf} = 14$ T.

We report optimized crystal growth conditions for the quaternary compound CuCrP$_2$S$_6$ by chemical vapor transport. Compositional and structural characterization of the obtained crystals were carried out by means of energy-dispersive X-ray spectroscopy and powder X-ray diffraction. CuCrP$_2$S$_6$ is structurally closely related to the $M_2$P$_2$S$_6$ family ($M$: transition metal), which contains several compounds that are under investigation as 2D magnets. As-grown crystals exhibit a plate-like, layered morphology as well as a hexagonal habitus. We present successful exfoliation of such as-grown crystals down to thicknesses of 2.8 nm corresponding to 4 layers. CuCrP$_2$S$_6$ crystallizes in the monoclinic space group $C2/c$. Magnetization measurements reveal an antiferromagnetic ground state with $T_\textrm{N} \approx 30$ K and a positive Curie-Weiss temperature in agreement with dominant ferromagnetic intralayer coupling. Specific heat measurements confirm this magnetic phase transition and the magnetic order is suppressed in an external magnetic field of about 6 T (8 T) applied parallel (perpendicular) to the $ab$ plane. At higher temperatures between 140-200 K additional broad anomalies associated with structural changes accompanying antiferroelectric ordering are detected in our specific heat studies., Comment: 12 pages, 11 figures, 46 references

We report a high-resolution angle-resolved photoemission (ARPES) study of the prototypical nearly-free-electron metal sodium. The observed mass enhancement is slightly smaller than that derived in previous studies. The new results on the lifetime broadening increase the demand for theories beyond the random phase approximation. Our results do not support the proposed strong enhancement of the scattering rates of the charge carriers due to a coupling to spin fluctuations. Moreover, a comparison with earlier electron energy-loss data on sodium yields a strong reduction of the mass enhancement of dipolar electron-hole excitations compared to that of monopole hole excitations, measured by ARPES., Comment: 5 pages, 6 figures

Linear acceleration emission is one of the mechanisms that might explain intense coherent emissions of radio pulsars. This mechanism is not well understood, however, because the effects of collective plasma response and nonlinear plasma evolution on the resulting emission power must be taken into account. In addition, details of the radio emission properties of this mechanism are unknown, which limits the observational verification of the emission model. By including collective and nonlinear plasma effects, we calculate radio emission power properties by the linear acceleration emission mechanism that occurs via the antenna principle for two instabilities in neutron star magnetospheres: 1) the relativistic streaming instability, and 2) interactions of plasma bunches. We used 1D electrostatic relativistic particle-in-cell simulations to evolve the instabilities self-consistently. From the simulations, the power properties of coherent emission were obtained by novel postprocessing of electric currents. We found that the total radio power by plasma bunch interactions exceeds the power of the streaming instability by eight orders of magnitude. The wave power generated by a plasma bunch interaction can be as large as $2.6\times10^{16}$ W. The number of bunch interactions that are required to explain the typical pulsar power, $10^{18}$-$10^{22}$ W, depends on how the coherent emissions of bunches are added up together. Although $\sim$$4\times (10^1-10^5)$ simultaneously emitting bunches are necessary for an incoherent addition of their radiation power, $\gtrsim 6-600$ bunches can explain the total pulsar power if they add up coherently. The radio spectrum of the plasma bunch is characterized by a flatter profile for low frequencies and by a power-law index up to $\approx-1.6 \pm 0.2$ for high frequencies. The plasma bunches simultaneously radiate in a wide range of frequencies., Comment: 19 pages, 13 figures, 1 table

The mineral linarite (PbCuSO$_4$(OH)$_2$) forms a monoclinic structure where a sequence of Cu(OH)$_2$ units forms a spin-$\frac{1}{2}$ chain. Competing ferromagnetic nearest-neighbor ($J_1$) and antiferromagnetic next-nearest-neighbor interactions ($J_2$) in this quasi-one-dimensional spin structure imply magnetic frustration and lead to magnetic ordering below $T_N =$2.8 K in a mutliferroic elliptical spin-spiral ground state. Upon the application of a magnetic field along the spin-chain direction, distinct magnetically ordered phases can be induced. We studied the thermal conductivity $\kappa$ in this material across the magnetic phase diagram as well as in the paramagnetic regime in the temperature ranges 0.07-1 K and 9-300 K. We found that in linarite the heat is carried mainly by phonons but shows a peculiar non-monotonic behavior in field. In particular, $\kappa$ is highly suppressed at the magnetic phase boundaries, indicative of strong scattering of the phonons off critical magnetic fluctuations. Even at temperatures far above the magnetically ordered phases, the phononic thermal conductivity is reduced due to scattering off magnetic fluctuations. The mean free path due to spin-phonon scattering ($l_{\text{spin-phonon}}$) was determined as function of temperature. A power law behavior was observed mainly above 0.5 K indicating the thermal activation of spin fluctuations. In the critical regime close to the saturation field, $l_{\text{spin-phonon}}$ shows a $1/T$ dependence. Furthermore, a magnon thermal transport channel was verified in the helical magnetic phase. We estimate a magnon mean free path which corresponds to about 1000 lattice spacings.

The precise crystal symmetry and hence the emergence of the electric polarization still remains an open question in the multiferroic materials $R$Mn$_2$O$_5$ ($R$ = rare-earth, Bi, Y). While previous diffraction studies have indicated that $R$Mn$_2$O$_5$ possesses the centro-symmetric space group P$bam$, an atomic displacement allowing for the electric polarization would require a non-centrosymmetric crystal symmetry. Our single crystal neutron diffraction experiments on TbMn$_2$O$_5$ provide direct evidence of a reduced crystallographic symmetry already above the magnetic and ferroelectric phase transitions and a change in magnetic order upon entering the ferroelectric phase. This is indicated through the presence of additional nuclear Bragg reflections that are otherwise forbidden for the space group P$bam$ but are in good agreement with the polar space group P$12_11$. It implies that the exchange-striction, which arises from a symmetric $S_i \cdot S_j$ spin coupling, is the dominating mechanism for the generation of the electric polarization in the commensurate magnetic phase of TbMn$_2$O$_5$. Furthermore, the commensurate magnetic reflections are in accordance with a quartile step spin-spiral along the $c$-axis. Therefore, the antisymmetric $S_i \times S_j$ exchange via the inverse Dzyaloshinskii-Moriya interaction contributes as well and becomes the leading term in the low temperature incommensurate spin-spiral magnetic phase. These new findings provide important information for the understanding of the complex interplay between the magnetic and the structural order throughout the $R$Mn$_2$O$_5$ series of type-II multiferroics., Comment: 27 pages, 6 figures, Supplemental Material attached

We present a $^{125}$Te nuclear magnetic resonance (NMR) study in the three-dimensional spin web lattice Cu$_3$TeO$_6$, which harbors topological magnons. The $^{125}$Te NMR spectra and the Knight shift $\mathcal{K}$ as a function of temperature show a drastic change at $T_\text{S}\sim 40$ K much lower than the N\'eel ordering temperature $T_\text{N}\sim 61$ K, providing evidence for the first-order structural phase transition within the magnetically ordered state. Most remarkably, the temperature dependence of the spin-lattice relaxation rate $T_1^{-1}$ unravels spin-gap-like magnetic excitations, which sharply sets in at $T^*\sim 75$ K, the temperature well above $T_\text{N}$. The spin gap behavior may be understood by weakly dispersive optical magnon branches of high-energy spin excitations originating from the unique corner-sharing Cu hexagon spin-1/2 network with low coordination number., Comment: 6 pages, 2 figures, published in Physical Review Research

Magnetic reconnection in solar flares can efficiently generate non-thermal electron beams. The energetic electrons can, in turn, cause radio waves through microscopic plasma instabilities as they propagate through the ambient plasma along the magnetic field lines. We aim at investigating the wave emission caused by fast moving electron beams (FEBs) with characteristic non-thermal electron velocity distribution functions (EVDFs) generated by kinetic magnetic reconnection: two-streaming EVDFs along the separatrices and in the diffusion region, and perpendicular crescent-shaped EVDFs closer to the diffusion region. For this purpose, we utilized 2.5D fully kinetic Particle-In-Cell (PIC) code simulations in this study. We found that:(1) the two-streaming EVDFs plus the background ions are unstable to electron/ion (streaming) instabilities which cause ion acoustic waves and Langmuir waves due to the net current. This can lead to multiple harmonic plasma emission in the diffusion region and the separatrices of reconnection. (2) The perpendicular crescent-shaped EVDFs can cause multiple harmonic electromagnetic electron cyclotron waves through the electron cyclotron maser instabilities in the diffusion region of reconnection. Our results are applicable to diagnose the plasma parameters which control magnetic reconnection in solar flares by means of radio waves observations., Comment: 12 figures, 3 tables. This article has been accepted for publication in The Astrophysical Journal (ApJ)

We report a high frequency/high magnetic field electron spin resonance (HF-ESR) spectroscopy study in the sub-THz frequency domain of the two representatives of the family of magnetic topological insulators (MnBi$_{2}$Te$_{4}$)(Bi$_{2}$Te$_{3}$)$_{n}$ with $n = 0$ and 1. The HF-ESR measurements in the magnetically ordered state at a low temperature of $T = 4$ K combined with the calculations of the resonance modes showed that the spin dynamics in MnBi$_{\text{4}}$Te$_{\text{7}}$ is typical for an anisotropic easy-axis type ferromagnet (FM) whereas MnBi$_{\text{2}}$Te$_{\text{4}}$ demonstrates excitations of an anisotropic easy-axis type antiferromagnet (AFM). However, by applying the field stronger than a threshold value $\sim 6$ T we observed in MnBi$_{\text{2}}$Te$_{\text{4}}$ a crossover from the AFM resonance modes to the FM modes which properties are very similar to the ferromagnetic response of MnBi$_{\text{4}}$Te$_{\text{7}}$. We attribute this remarkably unusual effect unexpected for a canonical easy-axis AFM, which, additionally, can be accurately reproduced by numerical calculations of the excitation modes, to the closeness of the strength of the AFM exchange and magnetic anisotropy energies which appears to be a very specific feature of this compound. Our experimental data evidences that the spin dynamics of the magnetic building blocks of these compounds, the Mn-based septuple layers (SLs), is inherently ferromagnetic featuring persisting short-range FM correlations far above the magnetic ordering temperature as soon as the SLs get decoupled either by introducing a nonmagnetic quintuple interlayer, as in MnBi$_{\text{4}}$Te$_{\text{7}}$, or by applying a moderate magnetic field, as in MnBi$_{\text{2}}$Te$_{\text{4}}$, which may have an effect on the surface topological band structure of these compounds.

Electromagnetic waves due to electron-positron clouds (bunches), created by cascading processes in pulsar magnetospheres, have been proposed to explain the pulsar radio emission. In order to verify this hypothesis, we utilized for the first time Particle-in-Cell (PIC-) code simulations to study the nonlinear evolution of electron-positron bunches in dependence on the relative drift speeds of electrons and positrons, on the initial plasma temperature, and on the initial distance between the bunches. For this sake, we utilized the PIC-code ACRONYM with a high-order field solver and particle weighting factor, appropriate to describe relativistic pair plasmas. We found that the bunch expansion is mainly determined by the relative electron-positron drift speed. Finite drift speeds were found to cause the generation of strong electric fields that reach up to $E \sim 7.5 \times 10^{5}$ V/cm ($E / (m_\mathrm{e} c \omega_\mathrm{p} e^{-1}) \sim 4.4$) and strong plasma heating. As a result, up to 15~\% of the initial kinetic energy is transformed into the electric field energy. Assuming the same electron- and positron-distributions we found that the fastest (in the bunch reference frame) particles of consecutively emitted bunches eventually overlap in the momentum (velocity) space. This overlap causes two-stream instabilities that generate (electrostatic) subluminal L-mode waves with electric field amplitudes reaching up to $E \sim 1.9\times 10^{4}$ V/cm ($E / (m_\mathrm{e} c \omega_\mathrm{p} e^{-1}) \sim 0.11$). We found that the interaction of electron-position bunches leads to plasma heating, to the generation of strong electric fields and of intense superluminal L-mode waves which, in principle, can be behind the observed electromagnetic emissions of pulsars in the radio wave range., Comment: 18 pages, 13 figures

Magnetic reconnection can convert magnetic energy into kinetic energy of non-thermal electron beams. Those accelerated electrons can, in turn, cause radio emission in astrophysical plasma environments such as solar flares via micro-instabilities. The properties of the electron velocity distribution functions (EVDFs) of those non-thermal beams generated by reconnection are, however, still not well understood. In particular properties that are necessary conditions for some relevant micro-instabilities. We aim at characterizing the EVDFs generated in 3D magnetic reconnection by means of fully kinetic particle-in-cell (PIC) code simulations. In particular, our goal is to identify the possible sources of free energy offered by the generated EVDFs and their dependence on the strength of the guide field. By applying a machine learning algorithm on the EVDFs, we find that: (1) electron beams with positive gradients in their 1D parallel (to the local magnetic field direction) velocity distribution functions are generated in both diffusion region and separatrices. (2) Electron beams with positive gradients in their perpendicular (to the local magnetic field direction) velocity distribution functions are observed in the diffusion region and outflow region near the reconnection midplane. In particular, perpendicular crescent-shaped EVDFs (in the perpendicular velocity space) are mainly observed in the diffusion region. (3) As the guide field strength increases, the number of locations with EVDFs featuring a perpendicular source of free energy significantly decreases. The formation of non-thermal electron beams in the field-aligned direction is mainly due to magnetized and adiabatic electrons, while in the direction perpendicular to the local magnetic field it is attributed to unmagnetized electrons., Comment: 11 figures, 1 table. This article has been published in the journal Physics of Plasmas (PoP)

The entanglement of charge density wave (CDW), superconductivity, and topologically nontrivial electronic structure has recently been discovered in the kagome metal $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) family. With high-resolution angle-resolved photoemission spectroscopy, we study the electronic properties of CDW and superconductivity in CsV$_3$Sb$_5$. The spectra around $\bar{K}$ is found to exhibit a peak-dip-hump structure associated with two separate branches of dispersion, demonstrating the isotropic CDW gap opening below $E_{\rm F}$. The peak-dip-hump lineshape is contributed by linearly dispersive Dirac bands in the lower branch and a dispersionless flat band close to $E_{\rm F}$ in the upper branch. The electronic instability via Fermi surface nesting could play a role in determining these CDW-related features. The superconducting gap of $\sim$0.4 meV is observed on both the electron band around $\bar{\Gamma}$ and the flat band around $\bar{K}$, implying the multiband superconductivity. The finite density of states (DOS) at $E_{\rm F}$ in the CDW phase are most likely in favor of the emergence of multiband superconductivity, particularly the enhanced DOS associated with the flat band. Our results not only shed light on the controversial origin of the CDW, but also offer insights into the relationship between CDW and superconductivity., Comment: 6 pages, 4 figures, PRL in press

We present FlexibleDecay, a tool to calculate decays of scalars in a broad class of BSM models. The tool aims for high precision particularly in the case of Higgs boson decays. In the case of scalar and pseudoscalar Higgs boson decays the known higher order SM QED, QCD and EW effects are taken into account where possible. The program works in a modified $\bar{\text{MS}}$ scheme that exhibits a decoupling property with respect to heavy BSM physics, with BSM parameters themselves treated in the $\bar{\text{MS}}/\bar{\text{DR}}$-scheme allowing for an easy connection to high scale tests for, e.g., perturbativity and vacuum stability, and the many observable calculations readily available in $\bar{\text{MS}}/\bar{\text{DR}}$ programs. Pure BSM effects are taken into account at the leading order, including all one-loop contributions to loop-induced processes. The program is implemented as an extension to FlexibleSUSY, which determines the mass spectrum for arbitrary BSM models, and does not require any extra configuration from the user. We compare our predictions for Higgs decays in the SM, singlet extended SM, type II THDM, CMSSM and MRSSM, as well as for squark decays in the CMSSM against a selection of publicly available tools. The numerical differences between our and other programs are explained. The release of FlexibleDecay officially deprecates the old effective couplings routines in FlexibleSUSY., Comment: 44 pages, 3 figures, 11 tables; version published in CPC

Triangular Heisenberg antiferromagnets are prototypes of geometric frustration, even if for nearest-neighbor interactions quantum fluctuations are not usually strong enough to destroy magnetic ordering: stronger frustration is required to stabilize a spin-liquid phase. On the basis of static magnetization and electron spin resonance measurements, we demonstrate the emergence of ${\tilde{j}_{\text{eff}}=\frac12}$ moments in the triangular-lattice magnet Na$_2$BaCo(PO$_4$)$_2$. These moments are subject to an extra source of frustration that causes magnetic correlations to set in far above both the magnetic ordering and Weiss temperatures. Corroborating the $\tilde{j}_{\text{eff}}=\frac12$ ground state, theory identifies ferromagnetic Kitaev exchange anisotropy as additional frustrating agent, altogether putting forward Na$_2$BaCo(PO$_4$)$_2$ as a promising Kitaev spin-liquid material., Comment: 6 pages, 4 figures (published version) + supplemental material (4 pages)

Fermi surfaces, three-dimensional (3D) abstract interfaces that define the occupied energies of electrons in a solid, are important for characterizing and predicting the thermal, electrical, magnetic, and optical properties of crystalline metals and semiconductors [1]. Angle-resolved photoemission spectroscopy (ARPES) is the only technique directly probing the Fermi surface by measuring the Fermi momenta (kF) from energy and angular distribution of photoelectrons dislodged by monochromatic light [2]. Existing electron analyzers are able to determine a number of kF-vectors simultaneously, but current technical limitations prohibit a direct high-resolution 3D Fermi surface mapping. As a result, no such datasets exist, strongly limiting our knowledge about the Fermi surfaces and restricting a detailed comparison with the widely available nowadays calculated 3D Fermi surfaces. Here we show that using a simpler instrumentation, based on the Fourier electron optics combined with a retardation field of the detector, it is possible to perform 3D-mapping within a very short time interval and with very high resolution. We present the first detailed experimental 3D Fermi surface recorded in the full Brillouin zone along the kz-direction as well as other experimental results featuring multiple advantages of our technique. In combination with various light sources, including synchrotron radiation, our methodology and instrumentation offer new opportunities for high-resolution ARPES in the physical and life sciences., Comment: 14 pages, 4 figures

Hybrid metallic nanoparticles encapsulated in oxide shells are currently intensely studied for plasmonic applications in sensing, medicine, catalysis, and photovoltaics. Here, we introduce a method for the synthesis of Au@Ag@SiO$_2$ cubes with a uniform silica shell of variable and adjustable thickness in the nanometer range; and we demonstrate their excellent, highly reproducible, and tunable optical response. Varying the silica shell thickness, we could tune the excitation energies of the single nanoparticle plasmon modes in a broad spectral range between 2.55 and 3.25\,eV. Most importantly, we reveal a strong coherent coupling of the surface plasmons at the silver-silica interface with the whispering gallery resonance at the silica-vacuum interface leading to a significant field enhancement at the encapsulated nanoparticle surface in the range of 100\,\% at shell thicknesses $t\,$$\simeq\,$20\,nm. Consequently, the synthesis method and the field enhancement open pathways to a widespread use of silver nanoparticles in plasmonic applications including photonic crystals and may be transferred to other non-precious metals.

The heterostructure consisting of the Mott insulator LaVO$_3$ and the band insulator SrTiO$_3$ is considered a promising candidate for future photovoltaic applications. Not only does the (direct) excitation gap of LaVO$_3$ match well the solar spectrum, but its correlated nature and predicted built-in potential, owing to the non-polar/polar interface when integrated with SrTiO$_3$, also offer remarkable advantages over conventional solar cells. However, experimental data beyond the observation of a thickness-dependent metal-insulator transition is scarce and a profound, microscopic understanding of the electronic properties is still lacking. By means of soft and hard X-ray photoemission spectroscopy as well as resistivity and Hall effect measurements we study the electrical properties, band bending, and band alignment of LaVO$_3$/SrTiO$_3$ heterostructures. We find a critical LaVO$_3$ thickness of five unit cells, confinement of the conducting electrons to exclusively Ti 3$d$ states at the interface, and a potential gradient in the film. From these findings we conclude on electronic reconstruction as the driving mechanism for the formation of the metallic interface in LaVO$_3$/SrTiO$_3$., Comment: 13 pages, 12 figures

Layered van~der~Waals materials of the family TaTMTe$_4$ (TM=Ir, Rh, Ru) are showing very interesting electronic properties. Here we report the synthesis, crystal growth and structural characterization of TaIrTe$_4$, TaRhTe$_4$, TaIr$_{1-x}$Rh$_{x}$Te$_4$ ($x = 0.06$; 0.14; 0.78; 0.92) and Ta$_{1+x}$Ru$_{1-x}$Te$_4$ single crystals. For Ta$_{1+x}$Ru$_{1-x}$Te$_4$ off-stoichiometry is shown. X-ray powder diffraction confirms that TaRhTe4 is isostructural to TaIrTe4. We show that all these compounds are metallic with diamagnetic behavior. Ta$_{1.26(2)}$Ru$_{0.75(2)}$Te$_{4.000(8)}$ exhibits an upturn in the resistivity at low temperatures which is strongly field dependent. Below $T \approx 4$K we observed signatures of the superconductivity in the TaIr$_{1-x}$Rh$_{x}$Te$_4$ compounds for $x = 0.92$. Magnetotransport measurements on all samples show weak magnetoresistance (MR) field dependence that is typically quadratic-in-field. However, for TaIr$_{1-x}$Rh$_{x}$Te$_4$ with $x\approx 0.78$, the MR has a linear term dominating in low fields that indicates the presence of Dirac cones in the vicinity of the Fermi energy. For TaRhTe$_4$ series the MR is almost isotropic. We have performed electronic structure calculations for isostructural TaIrTe$_4$ and TaRhTe$_4$ together with the projected total density of states. The main difference is appearance of the Rh-band close to the Fermi level.

We report an optimized chemical vapor transport method to grow single crystals of (Mn$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ where x = 0, 0.3, 0.5, 0.7 & 1. Single crystals up to 4\,mm\,$\times$\,3\,mm\,$\times$\,200\,$\mu$m were obtained by this method. As-grown crystals characterized by means of scanning electron microscopy, and powder x-ray diffraction measurements. The structural characterization shows that all crystals crystallize in monoclinic symmetry with the space group $C2/m$ (No. 12). We have further investigated the magnetic properties of this series of single crystals. The magnetic measurements of the all as-grown single crystals show long-range antiferromagnetic order along all crystallographic principal axes. Overall, the N\'eel temperature TN is non-monotonous, with increasing $Ni^{2+}$ doping the temperature of the antiferromagnetic phase transition first decreases from 80 K for pristine Mn$_2$P$_2$S$_6$ (x = 0) up to x = 0.5, and then increases again to 155 K for pure Ni$_2$P$_2$S$_6$ (x = 1). The magnetic anisotropy switches from out-of-plane to in-plane as a function of composition in (Mn$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ series. Transport studies under hydrostatic pressure on the parent compound Mn$_2$P$_2$S$_6$ evidence an insulator-metal transition at an applied critical pressure of ~22 GPa

We report optimized crystal growth conditions for the quarternary compound AgCrP$_2$S$_6$ by chemical vapor transport. Compositional and structural characterization of the obtained crystals were carried out by means of energy-dispersive X-ray spectroscopy and powder X-ray diffraction. AgCrP$_2$S$_6$ is structurally closely related to the $M_2$P$_2$S$_6$ family, which contains several compounds that are under investigation as 2D magnets. As-grown crystals exhibit a plate-like, layered morphology as well as a hexagonal habitus. AgCrP$_2$S$_6$ crystallizes in monoclinic symmetry in the space group $P2/a$ (No. 13). The successful growth of large high-quality single crystals paves the way for further investigations of low dimensional magnetism and its anisotropies in the future and may further allow for the manufacturing of few-layer (or even monolayer) samples by exfoliation., Comment: Submitted to Crystals; 10 pages, 5 figures, 26 references

The recent discovery of magnetic topological insulators has opened new avenues to explore exotic states of matter that can emerge from the interplay between topological electronic states and magnetic degrees of freedom, be it ordered or strongly fluctuating. Motivated by the effects that the dynamics of the magnetic moments can have on the topological surface states, we investigate the magnetic fluctuations across the (MnBi$_{\text{2}}$Te$_{\text{4}}$)(Bi$_{\text{2}}$Te$_{\text{3}}$)$_{\text{n}}$ family. Our paramagnetic electron spin resonance experiments reveal contrasting Mn spin dynamics in different compounds, which manifests in a strongly anisotropic Mn spin relaxation in MnBi$_{\text{2}}$Te$_{\text{4}}$ while being almost isotropic in MnBi$_{\text{4}}$Te$_{\text{7}}$. Our density-functional calculations explain these striking observations in terms of the sensitivity of the local electronic structure to the Mn spin-orientation, and indicate that the anisotropy of the magnetic fluctuations can be controlled by the carrier density, which may directly affect the electronic topological surface states.

Discoveries of ordered quantum states of matter are of great fundamental interest, and often lead to unique applications. The most well known example -- superconductivity -- is caused by the formation and condensation of pairs of electrons. A key property of superconductors is diamagnetism: magnetic fields are screened by dissipationless currents. Fundamentally, what distinguishes superconducting states from normal states is a spontaneously broken symmetry corresponding to long-range coherence of fermion pairs. Here we report a set of experimental observations in hole doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ which are not consistent with conventional superconducting behavior. Our specific-heat measurements indicate the formation of fermionic bound states when the temperature is lowered from the normal state. However, for $x \sim 0.8$, instead of the standard for superconductors, zero resistance and diamagnetic screening, for a range of temperatures, we observe the opposite effect: the generation of self-induced magnetic fields measured by spontaneous Nernst effect and muon spin rotation experiments. The finite resistance and the lack of any detectable diamagnetic screening in this state exclude the spontaneously broken symmetry associated with superconducting two-fermion correlations. Instead, combined evidence from transport and thermodynamic measurements indicates that the formation of fermionic bound states leads to spontaneous breaking of time-reversal symmetry above the superconducting transition temperature. These results demonstrate the existence of a broken-time-reversal-symmetry bosonic metal state. In the framework of a multiband theory, such a state is characterized by quartic correlations: the long-range order exists only for {\it pairs} of fermion pairs., Comment: 31 pages, 11 figures, supplemental information

We report the crystal growth by chemical vapor transport together with a thorough structural and magnetic characterization of the quasi-2D magnets (Fe$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ with $x_\textrm{Ni} = \textrm{0, 0.3, 0.5, 0.7, 0.9 & 1}$. As-grown crystals exhibit a layered morphology with weak van der Waals interactions between layers parallel to the crystallographic $ab$ plane of the monoclinic structure in the space group $C2/m$ (No. 12). Magnetization measurements reveal an antiferromagnetic ground state for all grown crystals. In the ordered state, the magnetization along different crystallographic directions is in agreement with an Ising anisotropy for $0 \leq x \leq 0.9$ and only for Ni$_2$P$_2$S$_6$ a different behavior is observed which is line with an anisotropic Heisenberg or XXZ model description. This striking abrupt change of anisotropy at a critical Ni concentration is coupled to a change in the width of the maximum in the magnetization around $T_\textrm{max}$. Furthermore, all intermediate compounds $0 < x \leq 0.9$ exhibit an anisotropic magnetization already in the paramagnetic state similar to the parent compound Fe$_2$P$_2$S$_6$., Comment: 11 pages, 8 figures, 48 references

Magnetic skyrmions are stable topological solitons with complex non-coplanar spin structures. Their nanoscopic size and the low electric currents required to initiate and control their motion has opened a new field of research, skyrmionics, that aims at using skyrmions as information carriers for data storage and manipulation. Recent advances in skyrmionics prompt for a thorough understanding of the detailed three-dimensional (3D) spin texture of a skyrmion including skyrmion-skyrmion interactions and their coupling to surfaces and interfaces. These properties crucially affect application-related aspects such as the stability and mobility of skyrmions in confined structures. To date, however, experimental techniques to measure the three-dimensional spin texture with nanometer resolution are largely missing. We therefore adapt holographic vector field electron tomography to the problem and report on the first quantitative reconstruction of the 3D spin texture of skyrmions with sub-10 nanometer resolution. The reconstructed textures reveal a variety of previously unseen local deviations from a homogeneous Bloch character within the skyrmion tubes (SkTs), details of the collapse of the skyrmion texture at surfaces, and a correlated modulation of the SkT in FeGe along their tube axes. The quantitative 3D data of the magnetic induction also allow to experimentally confirm some principles of skyrmion formation by deriving spatially resolved maps of the magnetic energy density across these magnetic solitons.

We studied the field dependent thermal conductivity ($\kappa$) of Na$_2$Co$_2$TeO$_6$, a compound considered as the manifestation of the Kitaev model based on the high-spin $d^7$ Co$^{2+}$ ions. We found that in-plane magnetic fields beyond a critical value $B_c \approx$~10 T are able to drastically enhance $\kappa$ at low temperatures, resulting in a double-peak structure of $\kappa(T)$ that closely resembles the behavior of $\alpha$-RuCl$_3$. This result suggests that heat transport in Na$_2$Co$_2$TeO$_6$ is primarily phononic, and it is strongly affected by scattering from magnetic excitations that are highly tunable by external fields. Interestingly, for magnetic fields $B // a$ (i.e., along the zigzag direction of the Co-Co bonds), there is an extended field range which separates the long-range magnetic order for $B\leq B_c\approx10$ T and the partially spin-polarized gapped high-field phase for $B\gtrsim 12$ T. The low-energy phonon scattering is particularly strong in this field range, consistent with the notion that the system becomes a quantum spin liquid with prominent spin fluctuations down to energies of no more than 2 meV.

BaFe$_2$S$_3$ is a quasi-one-dimensional antiferromagnetic insulator that becomes superconducting under hydrostatic pressure. The magnetic ordering temperature, $T_N$, as well as the presence of superconductivity have been found to be sample dependent. It has been argued that the Fe content may play a decisive role, with the use of 5%mol excess Fe being reportedly required during the synthesis to optimize the magnetic ordering temperature and the superconducting properties. However, it is yet unclear whether an Fe off-stoichiometry is actually present in the samples, and how it affects the structural, magnetic and transport properties. Here, we present a systematic study of compositional, structural and physical properties of BaFe$_{2+\delta}$S$_3$ as a function of the nominal Fe excess $\delta$. As $\delta$ increases, we observe the presence of an increasing fraction of secondary phases but no systematic change in the composition or crystal structure of the main phase. Magnetic susceptibility curves are influenced by the presence of magnetic secondary phases. The previously reported maximum of $T_N$ at $\delta$=0.1 was not confirmed. Samples with nominal $\delta$=0 present the lowest $T_N$ and the resistivity anomaly at the highest temperature $T^*$ while, for $\delta \geq 0.05$, both quantities and the transport gap are seemingly $\delta$-independent. Finally, we show that crystals free of ferromagnetic spurious phases can be obtained by remelting samples with nominal $\delta$=0.05 in a Bridgman process., Comment: 11 pages, 8 figures

In this work we revisit the phase diagram of Co-doped LaFeAsO using single crystals and thermodynamic methods. From magnetic susceptibility studies we track the doping evolution of the antiferromagnetic phase, revealing a continuous suppression of $T_\mathrm{N}$ up to 5$\%$ Co doping. In order to study the evolution of the so-called nematic phase, the temperature dependence of the lengths changes along the $a$ and $b$ orthorhombic directions, $\Delta L/L_0$, was determined by high-resolution capacitance dilatometry. The results clearly show a gradual reduction of the orthorhombic distortion $\delta$ and of $T_\mathrm{S}$ with increasing Co content up to 4.5$\%$, while it is completely suppressed for 7.5$\%$ Co. Bulk superconductivity was found in a small doping region around 6$\%$ Co content, while both $T_\mathrm{c}$ and the superconducting volume fraction rapidly drop in the neighbouring doping regime. Ultimately, no microscopic coexistence between the superconducting and magnetic phases can be assessed within our resolution limit, in sharp contrast with other iron-pnictide families, e.g., electron- and hole-doped BaFe$_2$As$_2$.

A number of possible pulsar radio emission mechanisms are based on streaming instabilities in relativistically hot electron-positron pair plasmas. At saturation the unstable waves can form, in principle, stable solitary waves which could emit the observed intense radio signals. We searched for the proper plasma parameters which would lead to the formation of solitons, investigated their properties and dynamics as well as the resulting oscillations of electrons and positrons possibly leading to radio wave emission. We utilized a one-dimensional version of the relativistic Particle-in-Cell code ACRONYM initialized with an appropriately parameterized one-dimensional Maxwell-J\"uttner velocity space particle distribution to study the evolution of the resulting streaming instability in a pulsar pair plasma. We found that strong electrostatic superluminal L-mode solitons are formed for plasmas with normalized inverse temperatures $\rho \geq 1.66$ or relative beam drift speeds with Lorentz factors $\gamma > 40$. The parameters of the solitons fulfill the wave emission conditions. For appropriate pulsar parameters the resulting energy densities of superluminal solitons can reach up to $1.1 \times 10^5$ erg$\cdot$cm$^{-3}$, while those of subluminal solitons reach only up to $1.2 \times 10^4$ erg$\cdot$cm$^{-3}$. Estimated energy densities of up to $7 \times 10^{12}$ erg$\cdot$cm$^{-3}$ suffice to explain pulsar nanoshots., Comment: 20 pages, 15 figures, 1 table

Symmetry breaking in topological matter has become in recent years a key concept in condensed matter physics to unveil novel electronic states. In this work, we predict that broken inversion symmetry and strong spin-orbit coupling in trigonal PtBi$_2$ lead to a type-I Weyl semimetal band structure. Transport measurements show an unusually robust low dimensional superconductivity in thin exfoliated flakes up to 126 nm in thickness (with $T_c \sim 275-400$~mK), which constitutes the first report and study of unambiguous superconductivity in a type-I Weyl semimetal. Remarkably, a Berezinskii-Kosterlitz-Thouless transition with $T_\text{BKT} \sim 310$~mK is revealed in up to 60 nm thick flakes, which is nearly an order of magnitude thicker than the rare examples of two-dimensional superconductors exhibiting such a transition. This makes PtBi$_2$ an ideal platform to study low dimensional and unconventional superconductivity in topological semimetals., Comment: to be published in Nano Letters

We report the results of the pressure-dependent measurements of the static magnetization and of the ferromagnetic resonance (FMR) of Cr$_2$Ge$_2$Te$_6$ to address the properties of the ferromagnetic phase of this quasi-two-dimensional van der Waals magnet. The static magnetic data at hydrostatic pressures up to 3.4 GPa reveal a gradual suppression of ferromagnetism in terms of a reduction of the critical transition temperature, a broadening of the transition width and an increase of the field necessary to fully saturate the magnetization $M_{\rm s}$. The value of $M_{\rm s} \simeq 3\mu_{\rm B}$/Cr remains constant within the error bars up to a pressure of 2.8 GPa. The anisotropy of the FMR signal continuously diminishes in the studied hydrostatic pressure range up to 2.39 GPa suggesting a reduction of the easy-axis type magnetocrystalline anisotropy energy (MAE). A quantitative analysis of the FMR data gives evidence that up to this pressure the MAE constant $K_{\rm U}$, although getting significantly smaller, still remains finite and positive, i.e. of the easy-axis type. Therefore, a recently discussed possibility of switching the sign of the magnetocrystalline anisotropy in Cr$_2$Ge$_2$Te$_6$ could only be expected at still higher pressures, if possible at all due to the observed weakening of the ferromagnetism under pressure. This circumstance may be of relevance for the design of strain-engineered functional heterostructures containing layers of Cr$_2$Ge$_2$Te$_6$.

We determine the phase diagram of LaFe$_{1-x}$Co$_x$AsO single crystals by using nuclear magnetic resonance (NMR). Up to a nominal doping of $x=0.03$, it follows the phase diagram for F-doped polycrystals. Above $x=0.03$, the F-doped samples become superconducting, whereas for Co-doping the structural and magnetic transitions can be observed up to $x=0.042$, and superconductivity occurs only for higher doping levels and with reduced transition temperatures. For dopings up to $x=0.056$, we find evidence for short-range magnetic order. By means of relaxation-rate measurements, we map out the magnetic fluctuations that reveal the interplay of nematicity and magnetism. Above the nematic ordering, the spin fluctuations in LaFe$_{1-x}$Co$_x$AsO are identical to those in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, suggesting that nematicity in LaFeAsO is a result of the fluctuating spin density wave as well.

Less common ligand coordination of transition-metal centers is often associated with peculiar valence-shell electron configurations and outstanding physical properties. One example is the Fe$^+$ ion with linear coordination, actively investigated in the research area of single-molecule magnetism. Here we address the nature of 3$d^9$ states for Cu$^{2+}$ ions sitting in the center of trigonal bipyramidal ligand cages in the quasi-two-dimensional honeycomb compound InCu$_{2/3}$V$_{1/3}$O$_3$, whose unusual magnetic properties were intensively studied in the recent past. In particular, we discuss the interplay of structural effects, electron correlations, and spin-orbit couplings in this material. A relevant computational finding is a different sequence of the Cu ($xz$, $yz$) and ($xy$, $x^2\!-\!y^2$) levels as compared to existing electronic-structure models, which has implications for the interpretation of various excitation spectra. Spin-orbit interactions, both first- and second-order, turn out to be stronger than previously assumed, suggesting that rather rich single-ion magnetic properties can be in principle achieved also for the 3$d^9$ configuration by properly adjusting the sequence of crystal-field states for such less usual ligand coordination., Comment: 6 pages, 2 figures, and 3 tables

Several important mechanisms that explain the coherent pulsar radio emission rely on streaming (or beam) instabilities of the relativistic pair plasma in a pulsar magnetosphere. However, it is still not clear whether a streaming instability by itself is sufficient to explain the observed coherent radio emission. Due to the relativistic conditions that are present in the pulsar magnetosphere, kinetic instabilities could be quenched. Moreover, uncertainties regarding specific model-dependent parameters impede conclusions concerning this question. We aim to constrain the possible parameter range for which a streaming instability could lead to pulsar radio emission, focusing on the transition between strong and weak beam models, beam drift speed, and temperature dependence of the beam and background plasma components. We solve a linear relativistic kinetic dispersion relation appropriate for pulsar conditions in a more general way than in previous studies, considering a wider parameter range. The analytical results are validated by comparison with relativistic kinetic particle-in-cell (PIC) numerical simulations. We obtain growth rates as a function of background and beam densities, temperatures, and streaming velocities while finding a remarkable agreement of the linear dispersion predictions and numerical simulation results in a wide parameter range. Monotonous growth is found when increasing the beam-to-background density ratio. With growing beam velocity, the growth rates firstly increase, reach a maximum and decrease again for higher beam velocities. A monotonous dependence on the plasma temperatures is found, manifesting in an asymptotic behaviour when reaching colder temperatures. We show that the generated waves are phase-coherent by calculating the fractional bandwidth. We provide an explicit parameter range of plasma conditions for efficient pulsar radio emission., Comment: 15 pages, 7 figures. An abridged version of the abstract is shown here. Accepted in A&A

We report resistive switching in voltage biased point contacts (PCs) based on series of van der Waals transition metals tellurides (TMTs) such as MeTe2 (Me=Mo, W) and TaMeTe4 (Me= Ru, Rh, Ir). The switching occurs between a low resistive "metallic-type" state, which is the ground state, and a high resistive "semiconducting-type" state by applying certain bias voltage (<1V), while reverse switching takes place by applying voltage of opposite polarity. The origin of the effect can be formation of domain in PC core by applying a bias voltage, when a strong electric field (about 10kV/cm) modifies the crystal structure and controls its polarization. In addition to the discovery of the switching effect in PCs, we also suggest a simple method of material testing before functionalizing them, which offers a great advantage in finding suitable novel substances. The new functionality of studied TMTs arising from switchable domains in submicron hetero-structures that are promising, e.g., for non-volatile resistive random access memory (RRAM) engineering., Comment: 8 pages, 4 figs., supplement with 8 figs. V2 some text revision

Hitherto, the discrete identification of quantum spin liquid phase, holy grail of condensed matter physics, remains a challenging task experimentally. However, the precursor of quantum spin liquid state may reflect in the spin dynamics even in the paramagnetic phase over a wide temperature range as conjectured theoretically. Here we report comprehensive inelastic light (Raman) scattering measurements on the Ir based double perovskite, Gd2ZnIrO6, as a function of different incident photon energies and polarization in a broad temperature range. Our results evidenced the spin fractionalization within the paramagnetic phase reflected in the emergence of a polarization independent quasi-elastic peak at low energies with lowering temperature. Also, the fluctuating scattering amplitude measured via dynamic Raman susceptibility increases with lowering temperature and decreases mildly upon entering into long-range magnetic ordering phase, below 23 K, suggesting the magnetic origin of these fluctuations. This anomalous scattering response is thus indicative of fluctuating fractional spin evincing the quantum spin liquid phase in a three-dimensional double perovskite system., Comment: 3 figures