arXiv subjects

G. Q. Zhao

Publications and source records attributed to G. Q. Zhao.

At least 19 recordsLinked to original sources

arXiv2026

Ion-Scale Waves Regulated by Plasma Beta and Cross Helicity: Observational Evidence for the Helicity Barrier in Imbalanced Solar Wind Turbulence

Ion-scale waves are believed to play an important role in heating of the solar corona and wind, though their generation mechanism remains unclear. Based on Parker Solar Probe observations, this Letter investigates the occurrence of ion-scale waves in the near-Sun solar wind with heliocentric distances between 0.1 and 0.2 au. Results show that the occurrence rate of left-handed polarized waves significantly depends on the plasma beta ($β$) and cross helicity ($σ_c$). The occurrence rate rapidly increases with decreasing $β$ and increasing $σ_c$. Overall, the occurrence rate exceeds 45$\%$ when $β<0.2$ and $σ_c > 0.7$ are satisfied. These observations are consistent with the direct predictions of the helicity barrier theory that suggests the generation of ion cyclotron waves via imbalanced magnetized turbulence.

astro-ph.SR↗

arXiv2025

Nature of the ferromagnet-paramagnet transition in Y$_{1-x}$Ca$_{x}$TiO$_{3}$

Neutron scattering, magnetometry, and muon spin rotation ($μ$SR) measurements were performed to investigate the magnetic order and spin dynamics across the ferromagnet-to-paramagnet transition in the hole-doped Mott insulator Y$_{1-x}$Ca$_x$TiO$_3$. We find that the transition proceeds through a volume-wise phase separation into ferromagnetic and paramagnetic regions. Spin fluctuations with a characteristic timescale of $\sim$ 0.1 $μ$s, as detected via $μ$SR, are observed to appear at Ca concentrations $x \geq 0.10$. The magnetic phase separation, accompanied by a modest dynamic response, represents a novel behavior in Mott systems near the loss of magnetic order. It is linked to a previously observed insulator-metal transition and the associated electronic phase separation into hole-poor Mott insulating and hole-rich metallic phases for $0 < x < 0.50$. In particular, the $x$-dependence of the paramagnetic volume fraction strongly correlates with that of the volume fraction of the hole-rich metallic phase. The spin-wave spectra reveal a doping-induced crossover from isotropic to two-dimensional anisotropic exchange interactions, reflecting substantial changes in the orbital state with increasing Ca content.

cond-mat.str-el↗

arXiv2025

Topology meets time-reversal symmetry breaking in FeSe$_{1-x}$Te$_{x}$ superconductor

Time-reversal symmetry breaking (TRSB) in magnetic topological insulators induces a Dirac gap in the topological surface state (TSS), leading to exotic phenomena such as the quantum anomalous Hall effect. Yet, the interplay between TRSB and topology in superconductors remains underexplored due to limited suitable materials. Here we employ zero-field muon spin relaxation ($μ$SR) as a sensitive probe of TRSB to map out the electronic phase diagrams of iron-chalcogenide superconductors FeSe$_{1-x}$Te$_{x}$. For the Te composition $x=0.64$ with the highest superconducting transition temperature $T_{\rm c}=14.5$ K, which is known to host a TSS and Majorana zero modes within vortices, we detect spontaneous magnetic fields below $T_{\rm c}$ distinct from a magnetic order. This signifies a TRSB superconducting state in the bulk, revealing the convergence of unconventional TRSB superconductivity with topologically nontrivial electronic structures in FeSe$_{1-x}$Te$_{x}$. Given the relatively high $T_{\rm c}$ and the tunability of the Fermi level through chemical substitution, iron-chalcogenide superconductors offer an intriguing platform for investigating the synergy between topological superconductivity and TRSB.

cond-mat.supr-con↗

arXiv2022

Magnetic Properties and Electronic Configurations of Mn Ions in the Diluted Magnetic Semiconductor Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ Studied by X-ray Magnetic Circular Dichroism and Resonant Inelastic X-ray Scattering

The magnetic properties and the electronic excitations of the new diluted magnetic semiconductor Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ have been studied by x-ray magnetic circular dichroism (XMCD) and resonant inelastic x-ray scattering (RIXS) at the Mn $L_{2,3}$ edge. The sum rule analysis of the XMCD spectra yields the net spin moment of $0.45μ_{\text{B}}$/Mn and the small orbital moment of $0.05μ_{\text{B}}$/Mn. This indicates that the Mn atoms are in the high-spin configurations of $d^{5}$, whereas the presence of competing ferromagnetic and antiferromagnetic interactions between the Mn ions reduces the net spin moment. RIXS spectra show broad peaks from 1 to 6 eV energy loss, which originate from the $d$-$d$ crystal field excitations of the Mn ions. Based on a comparison of the RIXS line shapes with those of Ga$_{1-x}$Mn$_{x}$As, we conclude that the ground state of Mn in Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ consists not only of the charge-transferred $3d^{5}\underline{L}$ electron configuration ($\underline{L}$: ligand hole) with weakly bound holes as in Ga$_{1-x}$Mn$_{x}$As, but also of the pure $3d^{5}$ configuration with free holes.

cond-mat.str-el↗

arXiv2021

Nature of the ferromagnetic-antiferromagnetic transition in Y$_{1-x}$La$_{x}$TiO$_{3}$

We explore the magnetically-ordered ground state of the isovalently-substituted Mott-insulator Y$_{1-x}$La$_{x}$TiO$_{3}$ for $x$ $\leq$ 0.3 via single crystal growth, magnetometry, neutron diffraction, x-ray magnetic circular dichroism (XMCD), muon spin rotation ($μ$SR) and small-angle neutron scattering (SANS). We find that the decrease in the magnetic transition temperature on approaching the ferromagnetic (FM) - antiferromagnetic (AFM) phase boundary at the La concentration $x_c$ $\approx$ 0.3 is accompanied by a strong suppression of both bulk and local ordered magnetic moments, along with a volume-wise separation into magnetically-ordered and paramagnetic regions. The thermal phase transition does not show conventional second-order behavior, since neither a clear signature of dynamic critical behavior nor a power-law divergence of the magnetic correlation length is found for the studied substitution range; this finding becomes increasingly obvious with substitution. Finally, from SANS and magnetometry measurements, we discern a crossover from easy-axis to easy-plane magneto-crystalline anisotropy with increasing La substitution. These results indicate complex changes in magnetic structure upon approaching the phase boundary.

cond-mat.str-el↗

arXiv2020

Magnetic helicity signature and its role in regulating magnetic energy spectra and proton temperatures in the solar wind

In a previous paper, we found that proton temperatures are clearly associated with the proton-scale turbulence in the solar wind, and magnetic helicity signature appears to be an important indicator in the association. Based on 15 years of in situ measurements, the present paper further investigates the magnetic helicity of solar wind turbulence and its role in regulating magnetic energy spectra and proton temperatures. Results show that the presence of the helicity signature is very common in solar wind turbulence at scales $0.3 \lesssim kρ_p \lesssim 1$, with $k$ being the wavenumber and $ρ_p$ the proton gyroradius. The sign of the helicity is mostly positive, indicating the dominance of right-handed polarization of the turbulence. The helicity magnitude usually increases with $k$ and $β_{{\parallel}p}$ (the proton parallel beta) when $kρ_p$ and $β_{{\parallel}p}$ are less than unity. As helicity magnitude increases, the power index of the energy spectrum becomes more negative, and the proton temperatures $T_{{\perp}p}$ and $T_{{\parallel}p}$ rise significantly, where $T_{{\perp}p}$ and $T_{{\parallel}p}$ are the perpendicular and parallel temperatures with respect to the background magnetic field. In particular, the rise of $T_{{\perp}p}$ is faster than $T_{{\parallel}p}$ when $β_{{\parallel}p} < 1$ is satisfied. The faster rise of $T_{{\perp}p}$ with the helicity magnitude may be interpreted as the result of the preferentially perpendicular heating of solar wind protons by kinetic Alfvén wave (KAW) turbulence.

physics.space-ph↗

arXiv2020

Observational evidence for solar wind proton heating by ion-scale turbulence

Based on in-situ measurements by Wind spacecraft from 2005 to 2015, this letter reports for the first time a clearly scale-dependent connection between proton temperatures and the turbulence in the solar wind. A statistical analysis of proton-scale turbulence shows that increasing helicity magnitudes correspond to steeper magnetic energy spectra. In particular, there exists a positive power-law correlation (with a slope $\sim 0.4$) between the proton perpendicular temperature and the turbulent magnetic energy at scales $0.3 \lesssim kρ_p \lesssim 1$, with $k$ being the wavenumber and $ρ_p$ being the proton gyroradius. These findings present evidence of solar wind heating by the proton-scale turbulence. They also provide insight and observational constraint on the physics of turbulent dissipation in the solar wind.

physics.space-ph↗

arXiv2020

Dependence of ion temperatures on alpha-proton differential flow vector and heating mechanisms in the solar wind

According to \emph{Wind} observations between June 2004 and May 2019, this Letter investigates the proton and alpha particle temperatures in the space of ($θ_d$, $V_d/V_A$) for the first time, where $θ_d$ and $V_d$ are the radial angle and magnitude of alpha$-$proton differential flow vector respectively, $V_A$ is the local Alfvén speed. Results show that the temperatures significantly depend on $θ_d$ as well as $V_d/V_A$. In case of low proton parallel beta ($β_{p{\parallel}} < 1$), it is found that the proton perpendicular temperature is clearly enhanced when $θ_d$ is small ($\lesssim 45^\circ$) and $V_d/V_A \gtrsim 0.5$. On the contrary, the perpendicular temperature of alpha particles is considerably enhanced when $θ_d$ is large ($\gtrsim 90^\circ$) or $V_d/V_A$ is sufficiently small. The maximum of proton parallel temperature takes place at $θ_d \sim 90^\circ$ accompanied by higher $β_{p{\parallel}}$ and by larger turbulence amplitude of magnetic fluctuations in inertial range. This study should present strong evidence for cyclotron resonance heating of protons and alpha particles in the solar wind. Other mechanisms including Landau resonance and stochastic heating are also proposed, which tend to have different ($θ_d$, $V_d/V_A$) spaces than cyclotron resonance heating.

physics.space-ph↗

arXiv2019

Effects of alpha-proton differential flow on proton temperature anisotropy instabilities in the solar wind: Wind observations

Plasma kinetic waves and alpha-proton differential flow are two important subjects on the topic of evolution of the solar wind. Based on the Wind data during 2005-2015, this paper reports that the occurrence of electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency significantly depends on the direction of alpha-proton differential flow Vd. As Vd rotates from anti-Sunward direction to Sunward direction, the occurrence rate of ECWs as well as the percentage of left-handed (LH) polarized ECWs decreases considerably. In particular, it is shown that the dominant polarization changes from LH polarization to right-handed polarization during the rotation. The investigation on proton and alpha particle parameters ordered by the direction of Vd further illustrates that large kinetic energies of alpha-proton differential flow correspond to high occurrence rates of ECWs. These results are well consistent with theoretical predictions for effects of alpha-proton differential flow on proton temperature anisotropy instabilities.

physics.space-ph↗

arXiv2019

Superconductivity in a unique type of copper oxide

The mechanism of superconductivity in cuprates remains one of the big challenges of condensed matter physics.High Tc cuprates crystallize into layered perovskite structure featuring copper oxygen octahedral coordination. Due to the Jahn Teller effect in combination with the strong static Coulomb interaction, the octahedra in high Tc cuprates are elongated along the c axis, leading to a 3dx2-y2 orbital at the top of the band structure wherein the doped holes reside.This scenario gives rise to two dimensional characteristics in high Tc cuprates that favor d wave pairing symmetry. Here we report superconductivity in a cuprate Ba2CuO4-y wherein the local octahedron is in a very exceptional compressed version.The Ba2CuO4-y compound was synthesized at high pressure at high temperatures, and shows bulk superconductivity with critical temperature Tc above 70 K at ambient conditions. This superconducting transition temperature is more than 30 K higher than the Tc for the isostructural counterparts based on classical La2CuO4. X-ray absorption measurements indicate the heavily doped nature of the Ba2CuO4-y superconductor. In compressed octahedron the 3d3z2-r2 orbital will be lifted above the 3dx2-y2 orbital, leading to significant three dimensional nature in addition to the conventional 3dx2-y2 orbital. This work sheds important new light on advancing our comprehensive understanding of the superconducting mechanism of high Tc in cuprate materials.

cond-mat.supr-con↗

arXiv2019

Possible cool prominence materials detected within interplanetary small magnetic flux ropes

Previous studies indicate that interplanetary small magnetic flux ropes (SMFRs) are manifestations of microflare-associated small coronal mass ejections (CMEs), and the hot material with high charge states heated by related microflares are found in SMFRs. Ordinary CMEs are frequently associated with prominence eruptions,and cool prominencematerialsare found within some magnetic clouds (MCs). Therefore, the predicted small CMEs may also be frequently associated with small prominence eruptions. In this work, we aim to search for cool prominence materials within SMFRs.We examined all the O5+ and Fe6+ fraction data obtained by the Advanced Composition Explorer spacecraft during 1998 to 2008 and found that 13 SMFRs might exhibit low-charge-state signatures of unusual O5+and/or Fe6+abundances.One of the 13 SMFRs also exhibited signatures of high ionic charge states. We also reported a SMFR with highFe6+ fraction, but the values of Fe6+is a little lower than the threshold defining unusualFe6+.However, the SDO/AIA observations confirmed that the progenitor CME of this SMFR is associated with a small eruptive prominence, and the observations also supported the prominence materials were embedded in the CME.These observations are at the edge of the capabilities of ACE/SWICS and it cannot be ruled out that they are solely caused by instrumental effects. If these observations are real, they provide new evidence for the conjecture that SMFRs are small-scale MCs but also imply that the connected small CMEs could be associated with flares and prominence eruptions.

physics.space-ph↗

arXiv2019

Small Interplanetary Magnetic Flux Rope

Small interplanetary magnetic flux ropes (SIMFRs) are often detected by space satellites in the interplanetary space near 1 AU. These ropes can be fitted by a cylindrically symmetric magnetic model. The durations of SIMFRsare usually <12 h, and the diameters of SIMFRsare <0.20 AU and show power law distribution. Most SIMFRs are observed in the typically slow solar wind (<500 km/s), and only several events are observed with high speed (>700 km/s). Some SIMFRs demonstrate abnormal heavy ion compositions, such as abnormally high He abundance, abnormally high average iron ionization, and enhanced O7+ abundance. These SIMFRs originate from remarkablyhot coronal origins. Approximately 74.5% SIMFRs exhibit counterstreamingsuprathermal electron signatures. Given their flux rope configuration, SIMFRs are potentially more effective for substorms. SIMFRs and magnetic clouds havemany similar observational properties but also show some different observations.These similar properties may indicate that SIMFRs are the interplanetary counterparts of small coronal mass ejections. Some direct bodies of evidence have confirmed that several SIMFRs areinterplanetary counterparts of CMEs. However, their different properties may imply that some SIMFRs haveinterplanetary origins. Therefore, one of the main aims of future research on SIMFRs is to determine whether SIMFRs originate from two different sources, that is, some events are formed in the solar coronal atmosphere, whereas others originate from the interplanetary space. Finally, in this study, we offer some prospects that shouldbe addressed in the future.

physics.space-ph↗

arXiv2018

On the generation mechanism of electromagnetic cyclotron waves in the solar wind: statistical results from Wind observations

Electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency are frequently observed in the solar wind, yet their generation mechanism is still an open question. Based on the Wind data during the years 2005$-$2015, this paper carries out a statistical study on plasma characteristics associated with the occurrence of ECWs. The probability density distributions (PDDs) of proton temperature anisotropy ($T_\perp/T_\parallel$) and proton parallel beta ($β_\parallel$) are investigated, where $\perp$ and $\parallel$ refer to perpendicular and parallel to the background magnetic field, respectively. The PDDs depend on solar wind types as well as wave polarizations, and those for ECWs with left-handed (LH) polarization exhibit considerable differences from the PDDs for ambient solar winds. The distributions of occurrence rates of LH ECWs in ($β_\parallel$, $T_\perp/T_\parallel$) space show a tendency that the occurrence rates increase with proton temperature anisotropies. The $β_\parallel$ with maximum of occurrence rates is near 0.1 when $T_\perp/T_\parallel > 1$ while it is around 1 when $T_\perp/T_\parallel < 1$. The presence of alpha$-$proton differential flow with large kinetic energy corresponds to a much high occurrence rate as well as the domination of LH polarization of ECWs. Based on these observations and existing theories, we propose that the proton cyclotron and parallel firehose instabilities with effects of alpha$-$proton differential flow are likely responsible for the local generation of LH ECWs in the solar wind. The generation mechanism of right-handed ECWs seems to be complicated and more discussions are needed in future researches.

physics.space-ph↗

arXiv2018

Single Crystal Growth and Spin Polarization Measurements of Diluted Magnetic Semiconductor (BaK)(ZnMn)2As2

Recently a new type diluted magnetic semiconductor (BaK)(ZnMn)2As2 (BZA) with high Cure temperature (Tc) was discovered showing independent spin and charge doping mechanism. This makes BZA a promising material for spintronics devices. Here we report for the first time the successful growth of BZA single crystal. An Andreev reflection junction that can be used to evaluate spin polarization was fabricated based on the BZA single crystal, a 66% spin polarization of the BZA single crystal was hence obtained by Andreev reflection spectroscopy analysis.

cond-mat.mtrl-sci↗

arXiv2017

Time-dependent occurrence rate of electromagnetic cyclotron waves in the solar wind: evidence for effect of alpha particles?

Previous studies revealed that electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency exist widely in the solar wind, and the majority of ECWs are left-handed (LH) polarized waves. Using the magnetic field data from the STEREO mission, this Letter carries out a survey of ECWs over a long period of 7 years, and calculates the occurrence rates of ECWs with different polarization senses. Results show that the occurrence rate is nearly a constant for the ECWs with right-handed polarization, but it varies significantly for the ECWs with LH polarization. Further investigation of plasma conditions reveals that the LH ECWs take place preferentially in a plasma characterized by higher temperature, lower density, and larger velocity. Some considerable correlations between the occurrence rate of LH ECWs and the properties of ambient plasmas are discussed. The present research may provide evidence for effect of alpha particles on generation of ECWs.

physics.space-ph↗

arXiv2017

Out-of-plane easy-axis in thin films of diluted magnetic semiconductor Ba1-xKx(Zn1-yMny)2As2

Single-phased, single-oriented thin films of Mn-doped ZnAs-based diluted magnetic semiconductor (DMS) Ba1-xKx(Zn1-yMny)2As2 (x = 0.03, 0.08; y = 0.15) have been deposited on Si, SrTiO3, LaAlO3, (La,Sr)(Al,Ta)O3, and MgAl2O4 substrates, respectively. Utilizing a combined synthesis and characterization system excluding the air and further optimizing the deposition parameters, high-quality thin films could be obtained and be measured showing that they can keep inactive-in-air up to more than 90 hours characterized by electrical transport measurements. In comparison with films of x = 0.03 which possess relatively higher resistivity, weaker magnetic performances, and larger energy gap, thin films of x = 0.08 show better electrical and magnetic performances. Strong magnetic anisotropy was found in films of x = 0.08 grown on (La,Sr)(Al,Ta)O3 substrate with their magnetic polarization aligned almost solely on the film growth direction.

cond-mat.mtrl-sci↗

arXiv2016

The effect of electron holes on cyclotron maser emission driven by horseshoe distributions

This Brief Communication presents a quantitative investigation for the effect of electron holes on electron-cyclotron maser (ECM) driven by horseshoe distributions. The investigation is based on an integrated distribution function for the horseshoe distributions with electron holes. Results show that the presence of electron holes can significantly enhance the ECM growth rate by 2-3 times in a very narrow waveband. The present study suggests that these electron holes probably are responsible for some fine structures of radiations, such as narrowband events in auroral kilometric radiation and solar microwave spikes.

physics.plasm-ph↗

arXiv2016

Electron Cyclotron Maser Emissions from Evolving Fast Electron Beams

Fast electron beams (FEBs) are common products of solar active phenomena. Solar radio bursts are an important diagnostic tool in the understanding of FEBs as well as the solar plasma environment in which they are propagating along solar magnetic fields. In particular, the evolutions of the energy spectrum and velocity distribution of FEBs due to the interaction with the ambient plasma and field when propagating can significantly influence the efficiency and property of their emissions. In this paper, we discuss some possible evolutions of the energy spectrum and velocity distribution of FEBs due to the energy loss processes and the pitch-angle effect caused by the magnetic field inhomogeneity, and analyze the effects of these evolutions on electron cyclotron maser (ECM) emission, which is one of the most important mechanisms of producing solar radio bursts by FEBs. The results show that the growth rates all decrease with the energy loss factor $Q$, but increase with the magnetic mirror ratio $σ$ as well as with the steepness index $δ$. Moreover, the evolution of FEBs also can significantly influence the fastest growing mode and the fastest growing phase angle. This leads to the change of the polarization sense of ECM emission. In particular, our results also reveal that FEB that undergoes different evolution processes will generate different types of ECM emission. We believe the present results to be very helpful on more comprehensive understanding of dynamic spectra of solar radio bursts.

astro-ph.SR↗