arXiv subjects

C. -W. Wang

Publications and source records attributed to C. -W. Wang.

5 recordsLinked to original sources

arXiv2026

Magnetism and spin dynamics of Na$_5$Yb(MoO$_4$)$_4$: A weakly interacting rare-earth stretched diamond lattice

We report a comprehensive investigation of the structural and magnetic properties of Na$_5$Yb(MoO$_4$)$_4$, a member of the stretched diamond magnetic lattice family. Neutron powder diffraction at 3.3~K confirms that the compound crystallizes in the tetragonal \textit{I4$_1$/a} space group, with a large interatomic separation of 6.33~Å between magnetic Yb ions forming a three-dimensional stretched diamond framework. Magnetic susceptibility and specific heat measurements reveal no evidence of long-range magnetic order down to 60~mK. The low-temperature magnetic behavior is governed by an effective $J_{\mathrm{eff}} = 1/2$ Kramers doublet ground state, well separated from excited crystal-field levels, arising from the distorted dodecahedral oxygen coordination of Yb$^{3+}$. Density functional theory calculations within the DFT+$U$ framework indicate that exchange interactions between Yb ions are negligibly small, consistent with the long O--Mo--O super-superexchange pathways. The temperature dependence of the specific heat exhibits signatures of gapped spin excitations, most likely originating from long-range dipolar correlations and further shaped by weak exchange interactions together with the strong single-ion anisotropy of the Yb moments. Muon spin relaxation measurements reveal persistent low-energy spin dynamics, indicating that dipolar correlations remain dynamic and are insufficient to stabilize static magnetic order down to 50~mK. These results identify Na$_5$Yb(MoO$_4$)$_4$ as a rare example of a dipolar quantum paramagnet in which single-ion physics and long-range dipolar interactions dominate, while exchange interactions are suppressed to the millikelvin energy scale.

cond-mat.str-el↗

arXiv2025

Complex magnetic phases and thermodynamics of CuB_2O_4

The copper metaborate CuB_2O_4(CBO) has been studied within the parametric space of magnetic field (B) and temperature (T), showing a series of distinct phases, including commensurate and incommensurate magnetic orders, as well as magnetic soliton phases. However, no single study has simultaneously demonstrated all these phases. moreover, the existence of additional phases below 2 K or above 2 T for B perpenducular to c has been scarcely explored. This work presents the first comprehensive construction of the complex B-T phase diagram for T = 0.1 to 25 K and B = 0 to 9 T (B perpenducular to c), utilizing thermodynamic probes, magnetic measurements, and neutron scattering on a single batch of CBO crystals. Our findings elucidate new magnetic phases and regimes. First-principles calculations help to gain a deeper understanding of this complex phase diagram. The emergence of various phases within such a narrow B and T regime is attributed to two primary mechanisms: the delicate competition between ferromagnetic and antiferromagnetic interactions, and the weak exchange and superexchange interactions within and between the Cu(A) and Cu(B) sublattices. This competition also leads to a magnetically frustrated ground state at absolute zero where the total magnetic entropy release from high to low temperatures is only 0.76 Rln2, lower than Rln2 expected for copper spin 1/2. Using CBO as a model system, the present work elucidates the intricate mechanisms underlying the formation of complex magnetic phases.

cond-mat.str-el↗

arXiv2025

Evidence of mini-jet emission in a large emission zone from a magnetically-dominated gamma-ray burst jet

The second brightest GRB in history, GRB230307A, provides an ideal laboratory to study the mechanism of GRB prompt emission thanks to its extraordinarily high photon statistics and its single episode activity. Here we demonstrate that the rapidly variable components of its prompt emission compose an overall broad single pulse-like profile. Although these individual rapid components are aligned in time across all energy bands, this overall profile conspires to show a well-defined energy-dependent behavior which is typically seen in single GRB pulses. Such a feature demonstrates that the prompt emission of this burst is from many individual emitting units that are casually linked in a emission site at a large distance from the central engine. Such a scenario is in natural consistency with the internal-collision-induced magnetic reconnection and turbulence framework, which invokes many mini-jets due to local magnetic reconnection that constantly appear and disappear in a global magnetically-dominated jet.

astro-ph.HE↗

arXiv2024

Magnetar emergence in a peculiar gamma-ray burst from a compact star merger

The central engine that powers gamma-ray bursts (GRBs), the most powerful explosions in the universe, is still not identified. Besides hyper-accreting black holes, rapidly spinning and highly magnetized neutron stars, known as millisecond magnetars, have been suggested to power both long and short GRBs. The presence of a magnetar engine following compact star mergers is of particular interest as it would provide essential constraints on the poorly understood equation of state for neutron stars. Indirect indications of a magnetar engine in these merger sources have been observed in the form of plateau features present in the X-ray afterglow light curves of some short GRBs. Additionally, some X-ray transients lacking gamma-ray bursts (GRB-less) have been identified as potential magnetar candidates originating from compact star mergers. Nevertheless, smoking gun evidence is still lacking for a magnetar engine in short GRBs, and the associated theoretical challenges have been raised. Here we present a comprehensive analysis of the broad-band prompt emission data of a peculiar, very bright GRB 230307A. Despite its apparently long duration, the prompt emission and host galaxy properties are consistent with a compact star merger origin, as suggested by its association with a kilonova. Intriguingly, an extended X-ray emission component shows up as the $γ$-ray emission dies out, signifying the likely emergence of a magnetar central engine. We also identify an achromatic temporal break in the high-energy band during the prompt emission phase, which was never observed in previous bursts and reveals a narrow jet with half opening angle of approximately $\sim 3.4^\circ (R_{GRB}/10^{15}~{cm})^{-1/2}$, where $R_{GRB}$ is the GRB prompt emission radius.

astro-ph.HE↗

arXiv2017

Striped Magnetic Ground State of the Kagome Lattice in Fe4Si2Sn7O16

We have experimentally identified a new magnetic ground state for the kagome lattice, in the perfectly hexagonal Fe2+ (3d6, S = 2) compound Fe4Si2Sn7O16. Representational symmetry analysis of neutron diffraction data shows that below T_N = 3.5 K, the spins on 2/3 of the magnetic ions order into canted antiferromagnetic chains, separated by the remaining 1/3 which are geometrically frustrated and show no long-range order down to at least T = 0.1 K. Moessbauer spectroscopy confirms that there is no static order on the latter 1/3 of the magnetic ions - i.e., they are in a liquid-like rather than a frozen state - down to at least 1.65 K. A heavily Mn-doped sample Fe1.45Mn2.55Si2Sn7O16 has the same magnetic structure. Although the propagation vector q = (0, 1/2 , 1/2 ) breaks hexagonal symmetry, we see no evidence for magnetostriction in the form of a lattice distortion within the resolution of our data. We discuss the relationship to partially frustrated magnetic order on the pyrochlore lattice of Gd2Ti2O7, and to theoretical models that predict symmetry breaking ground states for perfect kagome lattices.

cond-mat.str-el↗