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We report a study on the specific heat and heat transport of Tb$_2$Ti$_{2-x}$Zr$_x$O$_7$ ($x =$ 0, 0.02, 0.1, 0.2, and 0.4) single crystals at low temperatures and in high magnetic fields. The magnetic specific heat can be described by the Schottky contribution from the crystal-electric-field (CEF) levels of Tb$^{3+}$, with introducing Gaussian distributions of the energy split of the ground-state doublet and the gap between the ground state and first excited level. These crystals has an extremely low phonon thermal conductivity in a broad temperature range that can be attributed to the scattering by the magnetic excitations, which are mainly associated with the CEF levels. There is strong magnetic field dependence of thermal conductivity, which is more likely related to the field-induced changes of phonon scattering by the CEF levels than magnetic transitions or spin excitations. For magnetic field along the [111] direction, there is large thermal Hall conductivity at low temperatures which displays a broad peak around 8 T. At high fields up to 14 T, the thermal Hall conductivity decreases to zero, which supports its origin from either the spinon transport or the phonon skew scattering by CEF levels. The thermal Hall effect is rather robust with Zr doping up to 0.2 but is strongly weakened in higher Zr-doped sample., Comment: 9 pages, 8 figures, accepted for publication in Phys. Rev. B

$S$ = 1/2 Ising kagome antiferromagnets (KAFs) are rare in reality. In this work, we report the magnetic ground state and field-induced transitions of an $S$ = 1/2 Ising antiferromagnet with a stacked kagome geometry, Na$_5$Co$_{15.5}$Te$_6$O$_{36}$. Upon cooling, magnetic susceptibility measurement reveals three successive magnetic transitions at $T_1$ = 45.2 K, $T_2$ = 35.4 K, and $T_3 \sim$ 20 K. When the magnetic field is applied along the Ising $c$ axis, a strikingly anomalous initial magnetization lying outside the hysteresis loop is observed below $T_3$. Through the detailed characterizations, the magnetic field $vs$ temperature phase diagram is established. A partially disordered antiferromagnetic state is proposed below $T_1$, which becomes frozen below $T_3$. Na$_5$Co$_{15.5}$Te$_6$O$_{36}$ is therefore a promising candidate for $S$ = 1/2 Ising KAFs, and this unique composite structure may shed light on the exploratory path for $S$ = 1/2 Ising KAFs., Comment: 9 pages, 9 pictures, to be published in PRB

Rare-earth pyrochlores are known to exhibit exotic magnetic phenomena. We report a study of crystal growth and characterizations of a new rare-earth compound Er$_2$AlSbO$_7$, in which Al$^{3+}$ and Sb$^{5+}$ ions share the same positions with a random distribution. The magnetism are studied by magnetic susceptibility, specific heat and thermal conductivity measurements at low temperatures down to several tens of milli-kelvin. Different from the other reported Er-based pyrochlores exhibiting distinct magnetically ordered states, a spin-freezing transition is detected in Er$_2$AlSbO$_7$ below 0.37 K, which is primarily ascribed to the inherent structural disorder. A cluster spin-glass state is proposed in view of the frequency dependence of the peak position in the ac susceptibility. In addition, the temperature and field dependence of thermal conductivity indicates rather strong spin fluctuations which is probably due to the phase competition., Comment: 10 pages, 8 figures, accepted for publication Phys. Rev. Materials

The basic physics and recent progresses in theoretical and particle-in-cell (PIC) simulation studies of particle acceleration in multi-island magnetic reconnection are briefly reviewed. Particle acceleration in multi-island magnetic reconnection is considered a plausible mechanism for the acceleration of energetic particles in solar flares and the solar wind. Theoretical studies have demonstrated that such a mechanism can produce the observed power-law energy distribution of energetic particles if the particle motion is sufficiently randomized in the reconnection event. However, PIC simulations seem to suggest that the first-order Fermi acceleration mechanism is unable to produce a power-law particle energy distribution function in mildly relativistic multi-island magnetic reconnections. On the other hand, while simulations of highly relativistic reconnections appear to be able to produce a power-law energy spectrum, the spectral indices obtained are generally harder than the soft power-law spectra with indices $\sim -5$ commonly observed in the solar wind and solar flare events. In addition, the plasma heating due to kinetic instabilities in 3D magnetic reconnection may "thermalize" the power-law particles, making it even more difficult for multi-island reconnections to generate a power-law spectrum. We discuss the possible reasons that may lead to these problems., Comment: in publication

Kagom\'{e}-staircase compound Ni$_3$V$_2$O$_8$ is an attractive multiferroic material exhibiting rich phase diagrams. However, the magnetic properties and magnetic transitions have been studied only above 1.3 K. In this work, we study the thermal conductivity ($\kappa$) of Ni$_3$V$_2$O$_8$ single crystals at low temperatures down to 0.3 K and in magnetic fields up to 14 T. In zero field, the magnetic transitions from the low-temperature incommensurate (LTI) phase to the commensurate phase (C) and then to a second commensurate phase (C') yield anomalies in $\kappa(T)$ curves at $T\rm_{LC}$ = 3.7 K and $T\rm_{CC'}$ = 2.0 K, respectively, which indicates a significant phonon scattering by the critical spin fluctuations. When the field is applied along the $a$ axis, the field dependence of $\kappa$ displays four anomalies associated with different magnetic transitions and reveals an undetected magnetic state at subKelvin temperatures. In addition, the $\kappa(B)$ curves are found to depend not only on the history but also on the magnitude of applying field. When the field is applied along the $b$ axis, a high-field phase locating above the LTI and high-temperature incommensurate (HTI) phases is revealed., Comment: 9 pages, 12 figures, accepted for publication in Phys. Rev. B

We report a study of thermal conductivity and resistivity at ultra-low temperatures and in high magnetic fields for semi-metal materials TaAs$_2$ and NbAs$_2$ by using single crystal samples. The thermal conductivity is strongly suppressed in magnetic fields, having good correspondence with the large positive magnetoresistance, which indicates a dominant electronic contribution to thermal conductivity. In addition, not only the resistivity but also the thermal conductivity display clear quantum oscillations behavior at subKelvin temperatures and in magnetic fields up to 14 T. The most striking phenomenon is that the thermal conductivity show a $T^4$ behavior at very low temperatures, while the resistivity show a $T$-independent behavior. This indicates a strong violation of the Weidemann-Franz law and points to a non-Feimi liquid state of these materials., Comment: 6 pages, 4 figures, supplymentary material included, some minor correction

Magnetism of Cu$_2$(OH)$_3$Br single crystals based on a triangular lattice is studied by means of magnetic susceptibility, pulsed-field magnetization, and specific heat measurements. There are two inequivalent Cu$^{2+}$ sites in an asymmetric unit. Both Cu$^{2+}$ sublattices undergo a long-range antiferromagnetic (AFM) order at $T\rm_N$ = 9.3 K. Upon cooling, an anisotropy crossover from Heisenberg to $XY$ behavior is observed below 7.5 K from the anisotropic magnetic susceptibility. The magnetic field applied within the $XY$ plane induces a spin-flop transition of Cu$^{2+}$ ions between 4.9 T and 5.3 T. With further increasing fields, the magnetic moment is gradually increased but is only about half of the saturation of a Cu$^{2+}$ ion even in 30 T. The individual reorientation of the inequivalent Cu$^{2+}$ spins under field is proposed to account for the magnetization behavior. The observed spin-flop transition is likely related to one Cu site, and the AFM coupling among the rest Cu spins is so strong that the 30-T field cannot overcome the anisotropy. The temperature dependence of the magnetic specific heat, which is well described by a sum of two gapped AFM contributions, is a further support for the proposed scenario., Comment: 11 pages, 6 figures. Accepted by J. Phys.: Condens. Matter for publication

Magnetism of the $S$ = 1 Heisenberg antiferromagnets on the spatially anisotropic square lattice has been scarcely explored. Here we report a study of the magnetism, specific heat, and thermal conductivity on Ni[SC(NH$_2$)$_2$]$_6$Br$_2$ (DHN) single crystals. Ni$^{2+}$ ions feature an $S$ = 1 rectangular lattice in the $bc$ plane, which can be viewed as an unfrustrated spatially anisotropic square lattice. A long-range antiferromagnetic order is developed at $T \rm_N =$ 2.23 K. Below $T\rm_N$, an upturn is observed in the $b$-axis magnetic susceptibility and the resultant minimum might be an indication for the $XY$ anisotropy in the ordered state. A gapped spin-wave dispersion is confirmed from the temperature dependence of the magnetic specific heat. Anisotropic temperature-field phase diagrams are mapped out and possible magnetic structures are proposed., Comment: 10 pages, 9 figures, accepted for publication in Phys. Rev. B

Observations of the solar corona and the solar wind discover that the solar wind is unsteady and originates from the impulsive events near the surface of the Sun's atmosphere. How solar coronal activities affect the properties of the solar wind is a fundamental issue in heliophysics. We report a simulation and theoretical investigation of how nanoflare accelerated electron beams affect the kinetic-scale properties of the solar wind and generate coherent radio emission. We show that nanoflare-accelerated electron beams can trigger a nonlinear electron two stream instability, which generates kinetic Alfv\'en and whistler waves, as well as a non-Maxwellian electron velocity distribution function, consistent with observations of the solar wind. The plasma coherent emission produced in our model agrees well with the observations of Type III, J and V solar radio bursts. Open questions in the kinetic solar wind model are also discussed., Comment: Submitted to the Proceedings of 17th Annual International Astrophysics Conference, Santa Fe, 2018

Magnetic reconnection (MR) is considered as a major source of particle energization in astrophysical plasma. In the past, analysis of MR often assumes the magnetostatic condition, i.e. $\partial_t = 0$. We show that under the Sweet-Parker-Petschek framework, steady state is an over-constraint and is not achievable. On the other hand, the quasi-steady state defined as $\partial_t \mathbf{E} = 0$ but $\partial_t \mathbf{B} \neq 0$ or equivalently $\partial_t\mathbf{j}\neq 0$ better describes the asymptotic behaviour of MR without turbulence. The upper limit of MR rate for quasi-steady MR is found to be $\sim 1/3\sqrt{3} \sim 0.19$. The limit does not apply to impulsive or turbulent MR of which $\partial_t\mathbf{B} \neq 0$ and $\partial_t\mathbf{E} \neq 0$. In impulsive MR the rate can be higher or lower than 0.19 depending on the state of the turbulence. Our results may explain the apparent discrepancy in observations of solar flare MR rates. The analysis is independent of mass ratio and thus the results are applicable to pair plasma., Comment: updated manuscript

In space plasma, various effects of magnetic reconnection and turbulence cause the electron motion to significantly deviate from their Larmor orbits. Collectively these orbits affect the electron velocity distribution function and lead to the appearance of the "non-gyrotropic" elements in the pressure tensor. Quantification of this effect has important applications in space and laboratory plasma, one of which is tracing the electron diffusion region (EDR) of magnetic reconnection in space observations. Three different measures of agyrotropy of pressure tensor have previously been proposed, namely, $A\varnothing_e$, $D_{ng}$ and $Q$. The multitude of contradictory measures has caused confusion within the community. We revisit the problem by considering the basic properties an agyrotropy measure should have. We show that $A\varnothing_e$, $D_{ng}$ and $Q$ are all defined based on the sum of the principle minors (i.e. the rotation invariant $I_2$) of the pressure tensor. We discuss in detail the problems of $I_2$-based measures and explain why they may produce ambiguous and biased results. We introduce a new measure $AG$ constructed based on the determinant of the pressure tensor (i.e. the rotation invariant $I_3$) which does not suffer from the problems of $I_2$-based measures. We compare $AG$ with other measures in 2 and 3-dimension particle-in-cell magnetic reconnection simulations, and show that $AG$ can effectively trace the EDR of reconnection in both Harris and force-free current sheets. On the other hand, $A\varnothing_e$ does not show prominent peaks in the EDR and part of the separatrix in the force-free reconnection simulations, demonstrating that $A\varnothing_e$ does not measure all the non-gyrotropic effects in this case, and is not suitable for studying magnetic reconnection in more general situations other than Harris sheet reconnection., Comment: accepted by Phys. of Plasma

We report a systematic study of the structure, electric and magnetic properties of Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ single crystals with $x =$ 0.72 and 0.26. The DC and AC magnetic susceptibilities display anomalies with characteristic of the spin freezing. The crystals show ferroelectric transition at 40 K and 35 K ($T_{FE}$) for $x =$ 0.72 and 0.26, respectively, with a large value of 1400 $\mu$C/m$^2$ at 8 K for electric polarization ($P_c$) along the spin-chain ($c$-axis) direction. Interestingly, the electric polarization perpendicular to the chain direction ($P_{ab}$) can also be detected and has value of 450 and 500 $\mu$C/m$^2$ at 8 K for the $x =$ 0.72 and 0.26 samples, respectively. The specific heat and magnetic susceptibility show no anomaly around $T_{FE}$, which means that the electric polarization of these samples has no direct relationship with the magnetism. The X-ray diffraction and the Raman spectroscopy indicate that these samples may undergo Jahn-Teller distortions that could be the reason of electric polarization., Comment: 11 pages, 16 figures, accepted for publication in Phys. Rev. B

Whether turbulence induced anomalous resistivity (AR) can facilitate a fast magnetic reconnection in collisionless plasma is a subject of active debate for decades. Recent space observations suggest that the reconnection rate can be higher than the Hall-reconnection rate and turbulent dissipation is required. In this paper, using particle-in-cell simulations, we present a case study of how AR produced by Buneman instability accelerates magnetic reconnection. We first show that the AR/drag produced by Buneman instability in a thin electron current layer 1) can dissipate magnetic energy stored in the current layer through dissipation of the kinetic energy of electron beams; 2) The inhomogeneous drag caused by wave couplings spontaneously breaks the magnetic field lines and causes impulsive fast non-Hall magnetic reconnection on electron-scales with a mean rate reaching 0.6 $V_A$. We then show that a Buneman instability driven by intense electron beams around the x-point in a 3D magnetic reconnection significantly enhances the dissipation of the magnetic energy. Electron-scale magnetic reconnections driven by the inhomogeneous drag around the x-line enhances the reconnection electric field and the in-plane perpendicular magnetic field. About 40\% of the released magnetic energy is converted into the electron thermal energy by AR while 50\% is converted into the kinetic energy of the electron beams through the acceleration by the reconnection electric field. The enhanced magnetic energy dissipation is balanced by a net Poynting flux in-flow. About 10\% of the released magnetic energy is brought out by an enhanced Poynting flux out-flow. These results suggest that AR with sufficient intensity and electron-scale inhomogeneity can significantly accelerate magnetic reconnection., Comment: Physics of Plasma accepted

Continuous plasma coherent emission is maintained by repetitive Langmuir collapse driven by the nonlinear evolution of a strong electron two-stream instability. The Langmuir waves are modulated by solitary waves in the linear stage, and by electrostatic whistler waves in the nonlinear stage. Modulational instability leads to Langmuir collapse and electron heating that fills in cavitons. The high pressure is released via excitation of a short wavelength ion acoustic mode that is damped by electrons and that re-excites small-scale Langmuir waves---this process closes a feedback loop that maintains the continuous coherent emission., Comment: 1/30/2017, published in Proceedings of the National Academy of Sciences of the United States of America

The formation of the observed core-halo feature in the solar wind electron velocity distribution function is a long-time puzzle. In this letter based on the current knowledge of nanoflares we show that the nanoflare-accelerated electron beams are likely to trigger a strong electron two-stream instability that generates kinetic Alfv\'en wave and whistler wave turbulence, as we demonstrated in a previous paper. We further show that the core-halo feature produced during the origin of kinetic turbulence is likely to originate in the inner corona and can be preserved as the solar wind escapes to space along open field lines. We formulate a set of equations to describe the heating processes observed in the simulation and show that the core-halo temperature ratio of the solar wind is insensitive to the initial conditions in the corona and is related to the core-halo density ratio of the solar wind and to the quasi-saturation property of the two-stream instability at the time when the exponential decay ends. This relation can be extended to the more general core-halo-strahl feature in the solar wind. The temperature ratio between the core and hot components is nearly independent of the heliospheric distance to the sun. We show that the core-halo relative drift previously reported is a relic of the fully saturated two stream instability. Our theoretical results are consistent with the observations while new tests for this model are provided., Comment: Accepted by ApjL on 14 Oct

To understand the nature of anomalous resistivity in magnetic reconnection, we investigate turbulence-induced momentum transport and energy dissipation while a plasma is unstable to the Buneman instability in force-free current sheets. Using 3D particle-in-cell simulations, we find that the macroscopic effects generated by wave-particle interactions in Buneman instability can be approximately described by a set of electron fluid equations. We show that both energy dissipation and momentum transport along electric current in the current layer are locally quasi-static, but globally dynamic and irreversible. Turbulent drag dissipates both the streaming energy of the current sheet and the associated magnetic energy. The net loss of streaming energy is converted into the electron component heat conduction parallel to the magnetic field and increases the electron Boltzmann entropy. The growth of self-sustained Buneman waves satisfies a Bernoulli-like equation that relates the turbulence-induced convective momentum transport and thermal momentum transport. Electron trapping and de-trapping drive local momentum transports, while phase mixing converts convective momentum into thermal momentum. The drag acts like a micro-macro link in the anomalous heating processes. The decrease of magnetic field maintains an inductive electric field that re-accelerates electrons, but most of the magnetic energy is dissipated and converted into the component heat of electrons perpendicular to the magnetic field. This heating process is decoupled from the heating of Buneman instability in the current sheets. Ion heating is weak but ions plays an important role in assisting energy exchanges between waves and electrons. Cold ion fluid equations together with our electron fluid equations form a complete set of equations that describes the occurrence, growth, saturation and decay of the Buneman instability., Comment: To be published in Physics of Plasma in 07/2014

The observed sub-proton scale turbulence spectrum in the solar wind raises the question of how that turbulence originates. Observations of keV energetic electrons during solar quite-time suggest them as possible source of free energy to drive the turbulence. Using particle-in-cell simulations, we explore how free energy in energetic electrons, released by an electron two-stream instability drives Weibel-like electromagnetic waves that excite wave-wave interactions. Consequently, both kinetic Alfv\'enic and whistler waves are excited that evolve through inverse and forward magnetic energy cascades., Comment: 12 pages, 5 figures, Submitted to Physical Review Letters

The nonlinear development of the strong Buneman instability and the associated fast electron heating in thin current layers with $\Omega_e/\omega_{pe} <1$ are explored. Phase mixing of the electrons in wave potential troughs and a rapid increase in temperature are observed during the saturation of the instability. We show that the motion of trapped electrons can be described using a Hamiltonian formalism in the adiabatic approximation. The process of separatrix crossing as electrons are trapped and de-trapped is irreversible and guarantees that the resulting electron energy gain is a true heating process., Comment: 4 figs, 11 pages, Accepted by Physics of Plasma

Buneman instability is often driven in magnetic reconnection. Understanding how velocity shear in the beams driving the Buneman instability affects the growth and saturation of waves is relevant to turbulence, heating, and diffusion in magnetic reconnection. Using a Mathieu-equation analysis for weak cosine velocity shear together with Vlasov simulations, the effects of shear on the kinetic Buneman instability are studied in a plasma consisting of strongly magnetized electrons and cold unmagnetized ions. In the linearly unstable phase, shear enhances the coupling between oblique waves and the sheared electron beam, resulting in a wider range of unstable eigenmodes with common lower growth rates. The wave couplings generate new features of the electric fields in space, which can persist into the nonlinear phase when electron holes form. Lower hybrid instabilities simultaneously occur at $k_{\shortparallel}/k_{\perp} \sim \sqrt{m_e/m_i}$ with a much lower growth rate, and are not affected by the velocity shear., Comment: Accepted by Physics of Plasma

Using particle-in-cell simulations and kinetic theory, we explore the current-driven turbulence and associated electron heating in the dissipation region during 3D magnetic reconnection with a guide field. At late time the turbulence is dominated by the Buneman and lower hybrid instabilities. Both produce electron holes that co-exist but have very different propagation speeds. The associated scattering of electrons by the holes enhances electron heating in the dissipation region., Comment: 14 pages, 5 figures, submitted to GRL

The nonlinear development of streaming instabilities in the current layers formed during magnetic reconnection with a guide field is explored. Theory and 3-D particle-in-cell simulations reveal two distinct phases. First, the parallel Buneman instability grows and traps low velocity electrons. The remaining electrons then drive two forms of turbulence: the parallel electron-electron two-stream instability and the nearly-perpendicular lower hybrid instability. The high velocity electrons resonate with the turbulence and transfer momentum to the ions and low velocity electrons., Comment: Accepted by PRL

If Gamma-ray Bursts (GRBs) are generally associated with supernovae like 1998bw, a relatively wide intrinsic luminosity function is implied, which indicates the existence of a large amount of undetected dim bursts, and a much higher event-rate than is often assumed. If it is assumed that the intrinsic luminosity function of GRBs is a power-law: ${\phi (L)} \propto {L^{-\beta}}$ ($\beta > 0$, $L_{min} \leq L \leq L_{max}$), data from the BATSE 4B catalog can be used to constrain slope index $\beta$ and the dynamic range width Log ${L_{max}\over L_{min}}$. Using a K-S test comparison with the observational Log $N$ - Log $P$, we find constraints on the GRB fireball model, GRB jets, and the possible GRB contribution to cosmic gamma-ray background. We find the acceptable dynamic range for $10^2

Recent optical afterglow observations of gamma-ray bursts indicate a setting and distance scale that many relate to star-formation regions. In this paper, we use $$ and a set of artificial trigger thresholds to probe several potential GRB source density evolutionary scenarios. In particular, we compare a uniform subset of BATSE 4B data to cosmological scenarios where GRBs evolve as the comoving density, the star formation rate, the QSO rate, and the SN Type Ic rate. Standard candle bursts with power-law spectra and a universe without vacuum energy were assumed. Our results significantly favor a comoving density model, implying that GRB source density evolution is weaker than expected in these evolutionary scenarios. GRB density might still follow star-formation rates given proper concurrent GRB luminosity evolution, significant beaming, significant error in standard candle assumptions, or were a significant modification of star formation rate estimates to occur., Comment: 12 pages, 4 figures, accepted by ApJ