Spin-flip Transitions Research Articles

On the general nature of 21-cm-Lyman α emitter cross-correlations during reionization

ABSTRACT We explore how the characteristics of the cross-correlation functions between the 21-cm emission from the spin-flip transition of neutral hydrogen (H $\scriptstyle \rm I $) and early Lyman α (Ly α) radiation emitting galaxies (Ly α emitters, LAEs) depend on the reionization history and topology and the simulated volume. For this purpose, we develop an analytic expression for the 21-cm-LAE cross-correlation function and compare it to results derived from different astraeus and 21cmfast reionization simulations covering a physically plausible range of scenarios where either low-mass (≲ 109.5 M⊙) or massive (≳ 109.5 M⊙) galaxies drive reionization. Our key findings are: (i) the negative small-scale (≲ 2 cMpc) cross-correlation amplitude scales with the intergalactic medium’s (IGM) average H $\scriptstyle \rm I $ fraction (〈χH i〉) and spin-temperature weighted overdensity in neutral regions (〈1 + δ〉H i); (ii) the inversion point of the cross-correlation function traces the peak of the size distribution of ionized regions around LAEs; (iii) the cross-correlation amplitude at small scales is sensitive to the reionization topology, with its anticorrelation or correlation decreasing the stronger the ionizing emissivity of the underlying galaxy population is correlated to the cosmic web gas distribution (i.e. the more low-mass galaxies drive reionization); (iv) the required simulation volume to not underpredict the 21-cm-LAE anticorrelation amplitude when the cross-correlation is derived via the cross-power spectrum rises as the size of ionized regions and their variance increases. Our analytic expression can serve two purposes: to test whether simulation volumes are sufficiently large, and to act as a fitting function when cross-correlating future 21-cm signal Square Kilometre Array and LAE galaxy observations.

Unraveling the magnetic structure of YbNiSn single crystal via crystal growth and neutron diffraction

Neutron and X-ray diffraction experiments were performed on the ternary intermetallic compound YbNiSn, formerly categorized as a ferromagnetic Kondo compound. At zero field, an increase in scattering intensity was observed on top of allowed and forbidden nuclear reflections below Tc, breaking the reflection condition of the crystal symmetry Pnma. This indicates that the magnetic structure of YbNiSn is antiferromagnetic-type, rather than the previously proposed simple collinear ferromagnetic structure. Temperature dependence of the scattering intensity of the 011 reflection confirmed the magnetic ordering at 5.77(2) K. No incommensurate satellite reflection was observed at 2.5 K. By applying external magnetic field of 1 T along the a axis, the magnetic intensity at the nuclear-forbidden 001 position was suppressed, while a slight enhancement at the nuclear-allowed 002 position was observed. This suggests a spin-flip transition under the external magnetic field along the a axis in YbNiSn. The proposed magnetic structures at zero field and 1 T correspond to the magnetic space groups of Pn'm'a and Pnm'a', respectively. The piezomagnetic effect and the switch between the two magnetic space groups by the external stress, which could be detected by the anomalous Hall effect, are proposed.

Biaxial ([110]) strain influence on the n-type half-metallicity and Curie temperature of Tc-doped Janus MoSSe monolayer

2D materials having stable half-metallic (HM) ferromagnetism are pivotal for multifunctional spintronic devices such as non-volatile random access magnetic memories and magnetic sensors. Using ab-initio calculations, we theoretically demonstrate that the electron-doped (Tc-doping at Mo site) Janus MoSSe monolayer (ML) is an n-type HM ferromagnetic (FM) with a charge carrier density of 2.9 × 1012 cm−2, which is thermally, mechanically, and dynamically stable. In addition, the large HM energy band gap of 1.63 eV is high enough to establish the long spin mean free path, high spin-filtering performance, and to avoid thermally excited spin-flip transition. Simultaneously, the calculated Curie temperature (TC) using a classical 2D Heisenberg model is 175 K, which indicates that Tc-doped MoSSe ML could be a stable FM structure. Interestingly, it is revealed that TC approaches to room temperature value for Fe 3d transition metals doping at Mo site, which makes the Fe-doped MoSSe ML highly desired for device application. Moreover, a direct-to-indirect band gap transition occurs in the pristine MoSSe ML for a biaxial ([110]) compressive/tensile (comp./tens.) strain of ≥−/+2%. It is also predicted that the n-type HM nature of the Tc-doped ML is very sensitive against applied strains and can be preserved under the -3% comp. to +2% tens. strain range. Along with this, the system undergoes a transition from FM to a non-magnetic state at a critical tensile strain of ＋ 5%, while the FM state remains robust under applied comp. strains. Correspondingly, it is also proposes that TC can be enhanced when a moderate compr. strain is adjusted. Hence, the present work are supposed to trigger further experimental studies to probe HM FM behavior in various Janus MLs by the combined effect of electron doping and strain engineering.

Structural and magnetic phase transitions in Fe3Ge : A neutron diffraction study

A detailed neutron diffraction study of a stoichiometric ${\mathrm{Fe}}_{3}\mathrm{Ge}$ alloy with different initial states in a wide temperature range (up to 1000 K) made it possible to accurately reveal the features of magnetic and structural phase transformations in it. The use of complementary x-ray diffraction and analysis of neutron diffraction patterns by the Rietveld method allowed reliably separating the nuclear and magnetic contributions to the intensity of the diffraction peaks and performing refinement of the ${\mathrm{Fe}}_{3}\mathrm{Ge}$ magnetic state characteristics. Both main phases appearing in ${\mathrm{Fe}}_{3}\mathrm{Ge}$ are ferromagnetic with ${T}_{\mathrm{C}}=629$ K (hexagonal, $D{0}_{19}$) and ${T}_{\mathrm{C}}=714$ K (cubic, $L{1}_{2}$). In $D{0}_{19}$, the presence of a spin-flip transition (${T}_{\mathrm{sf}}=385$ K) was confirmed and the temperature dependence of the components of magnetic moment along the hexagonal axis and in the basal plane was obtained. It was shown that the transformation between ordered $L{1}_{2}$ and $D{0}_{19}$ structural states, predicted by the equilibrium-phase diagram, includes three steps: two diffusional stages ($L{1}_{2}\ensuremath{\rightarrow}A1, A3\ensuremath{\rightarrow}D{0}_{19}$) and one displacive stage ($A1\ensuremath{\rightarrow}A3$). The obtained structural data suggest that in Fe-Ge, as well as in Fe-Ga alloy, direct transitions between ordered phases are impossible. They should include a transition between disordered states.

Coupled Potential Energy Surfaces Strongly Impact the Lowest-Energy Spin-Flip Transition in Six-Coordinate Nickel(II) Complexes.

Luminescent complexes of earth-abundant first-row transition metals are of renewed, broad interest due to their spectroscopic and photochemical properties as well as emerging applications. New strong-field polypyridine ligands have led to six-coordinate 3d3 chromium(III) complexes with intense spin-flip luminescence in solution at room temperature. The ground and emissive states both arise from the (t2)3 electron configuration involving the dπ levels (O point group symmetry labels). Pseudoctahedral 3d8 nickel(II) complexes with such strong ligands are a priori also promising candidates for spin-flip luminescence. In contrast, the relevant electron configurations involve the dσ orbitals and (e)2 configurations. We have prepared the known nickel(II) complexes [Ni(terpy)2]2+, [Ni(phen)3]2+, and [Ni(ddpd)2]2+ as well as the novel complexes [Ni(dgpy)2]2+ and [Ni(tpe)2]2+ forming a series with increasing ligand field strengths (terpy = 2,2':6',2″-terpyridine; phen = 1,10-phenanthroline; ddpd = N,N'-dimethyl-N,N'-dipyridine-2-ylpyridine-2,6-diamine; dgpy = 2,6-diguanidylpyridine; tpe = 1,1,1-tris(pyrid-2-yl)ethane). The lowest-energy singlet and triplet excited states of these nickel(II) complexes are analyzed based on absorption spectra using ligand field theory and CASSCF-NEVPT2 calculations for vertical transition energies and a model based on coupled potential energy surfaces, leading to calculated absorption spectra in good agreement with the experimental data. No photoluminescence signal was observed in the wavelength ranges identified through the analyses of the absorption spectra. The models provide insight into key differences between the nickel(II) complexes and their strongly luminescent chromium(III) analogues.

Quasiparticle random-phase approximation calculations forM1transitions with the noniterative finite-amplitude method and application to neutron radiative capture cross sections

We derive the equations of quasiparticle random-phase approximation (QRPA) based on the finite amplitude method (FAM) with the Hartree-Fock+BCS (HF+BCS) single-particle states, and calculate the magnetic dipole (M1) transition for deformed gadolinium isotopes. Our QRPA calculation shows both large spin-flip transitions in the 5 to 10 MeV excitation energy and the low energy orbital transition that would correspond to the M1 scissors mode observed in nuclear experiments. Then, we calculate neutron capture reactions based on the statistical Hauser-Feshbach theory with the photoabsorption cross sections given by QRPA. We find that the capture cross section is enhanced due to the contribution from the low energy M1 transition although the calculated capture cross section still underestimates the experimental data. This issue in the calculated capture cross section could be improved by uncertainties of low energy E1 transition neglected in our QRPA calculation.

Effect of Co doping on structural and magnetic properties of kagome like La based cuprate francisite Cu3La(SeO3)2O2Cl

The effects of Co doping on the structural and magnetic properties of a frustrated layered rare earth based synthetic analog of mineral francisite [Cu3La(SeO3)2O2Cl;(LaCufr)] have been investigated experimentally with a prior theoretical input. We have estimated intra-layer and inter-layer exchange interactions from density functional theory, which reveals the strong antiferromagnetic (AFM) nature of the studied compounds. 10 at. % Co-doped CuLafr confirms incorporation of Co2+ at the Cu2+ site with the local environment around dopant Co nearly similar around that of host Cu. An observed field-induced meta-magnetic transition and a spin reorientation transition have been interpreted considering magnetic exchange interactions. Co-doped LaCufr exhibits a decrease in the AFM ordering temperature and the critical field corresponding to a meta-magnetic spin-flip type transition. The competition and modulation among different intra-layer exchange interactions upon incorporation of Co into the Cu site may lead to such situations. The evidence of spin–phonon coupling from Raman analysis in both compounds is noteworthy and enriches their magnetic properties.

Cs2KMnCl6: A possible half-metallic double perovskite for spintronics

Materials with 100% spin polarisation, high-speed conduction electrons, high Curie temperatures, and ferromagnetic ground state are known for spintronics. Double perovskites (DPs) with these features are becoming popular nowadays. Herein, introducing another halide-based DP, we determined that Cs2KMnCl6 is a robust intrinsic 3d ferromagnetic half-metal with a 4 μB magnetic moment per unit cell. It exhibits the metallic band for the spin-up configuration, enabling their smooth transport, together with a massive gap of ∼4.51 eV for the spin-down states, thus protecting it from spin–flip transitions. The calculated structural and elastic stability criteria substantiate the stability of the cubic Cs2KMnCl6 double perovskite with a F m 3 ¯ m symmetry. The 3d states of Mn mainly contribute to the robust ferromagnetism above room temperature and show Tc ∼ 534 K. The different Fermi surface topologies within the Brillouin zone establish its conducting nature for up-spin channels and favor the spin-polarized transport. Moreover, the spin-polarized dielectric constant and absorption coefficient also evidenced the half-metallic characteristics. Therefore, our findings offer a novel DP system for investigating spintronic application.

Quantum and Semiclassical Dynamics of Nonadiabatic Electronic Excitation of C(3P) to C1D) by Hyperthermal Collisions with N2.

The dynamics and kinetics of nonadiabatic excitation of C(3P) to C(1D) induced by hyperthermal collisions with N2 molecules are investigated using a quantum mechanical and two semiclassical nonadiabatic methods. The full-dimensional interaction potential energy surfaces and spin-orbit coupling, which facilitates the spin-forbidden process, are represented by a recently constructed diabatic potential energy matrix. The multistate quantum dynamics for selected partial waves found small transition probabilities due to the weak spin-orbit coupling. The spin-flip transition is the most favored near the threshold due to effective curve crossing. Strong oscillations are also found in the probabilities, which are attributable to resonances supported by the deep well in the singlet-state potential. Vibrational state-specified rate coefficients are reported from J-shifted quantum dynamics calculations, and they follow the Arrhenius form. Vibrational excitation in the N2 collision partner is found to increase the excitation rate at low temperatures, but the trend is reversed at high temperatures. The two semiclassical methods qualitatively reproduce the quantum rate coefficients.

Using parity-nonconserving spin-spin coupling to measure the Tl nuclear anapole moment in a TlF molecular beam

An experiment utilizing a TlF molecular beam is being developed by the CeNTREX collaboration to search for hadronic interactions that violate both time-reversal ($\mathcal{T}$) and parity ($\mathcal{P}$) invariance. Here, we propose to use the same beam to look for a $\mathcal{T}$-invariance conserving but $\mathcal{P}$-nonconserving (PNC) effect induced by the anapole moment of the Tl nucleus, via a vector coupling of the two nuclear spins in TlF. To measure the nuclear anapole moment, the dc electric and magnetic fields in CeNTREX are replaced by rf fields resonant with a nuclear spin-flip transition. We adapt the relativistic coupled-cluster method in combination with relativistic density functional theory for the calculation of the molecular PNC spin-spin vector coupling constant that links the experimental signal with the anapole moment. The value of the $\mathcal{P}$-conserving spin-spin coupling constant calculated within the same approach is found to be in good agreement with available experimental data.

Spin-flip-driven anomalous Hall effect and anisotropic magnetoresistance in a layered Ising antiferromagnet

The influence of magnetocrystalline anisotropy in antiferromagnets is evident in a spin flip or flop transition. Contrary to spin flops, a spin-flip transition has been scarcely presented due to its specific condition of relatively strong magnetocrystalline anisotropy and the role of spin-flips on anisotropic phenomena has not been investigated in detail. In this study, we present antiferromagnet-based functional properties on an itinerant Ising antiferromagnet Ca0.9Sr0.1Co2As2. In the presence of a rotating magnetic field, anomalous Hall conductivity and anisotropic magnetoresistance are demonstrated, the effects of which are maximized above the spin-flip transition. Moreover, a joint experimental and theoretical study is conducted to provide an efficient tool to identify various spin states, which can be useful in spin-processing functionalities.

Zeeman-Sisyphus deceleration for heavy molecules with perturbed excited-state structure

We demonstrate and characterize Zeeman-Sisyphus (ZS) deceleration of a beam of ytterbium monohydroxide. Our method uses a combination of large magnetic fields ($\ensuremath{\sim}2.5$ T) and optical spin-flip transitions to decelerate molecules while scattering only $\ensuremath{\sim}10$ photons per molecule. We study the challenges associated with the presence of internal molecular perturbations among the excited electronic states and discuss the methods used to overcome these challenges, including a modified ZS decelerator using microwave and optical transitions.

Thermal Hall effect in a van der Waals triangular magnet FeCl2

Thermal transport is a pivotal probe for studying low-energy, charge-neutral quasiparticles in insulating magnets. In this Letter, we report an observation of large magnetothermal conductivity and thermal Hall effect (THE) in a van der Waals antiferromagnet ${\mathrm{FeCl}}_{2}$. The magnetothermal conductivity reaches over $\ensuremath{\sim}700%$, indicating strong magnon-phonon coupling. Furthermore, we find an appreciable thermal Hall signal which changes sign concurrently with the spin-flip transition from the antiferromagnetic state to the polarized ferromagnetic state. Our theoretical calculations suggest that, in addition to the Berry curvature induced at the anticrossing points of the hybridized magnon and acoustic phonon modes of ${\mathrm{FeCl}}_{2}$, other mechanisms are needed to account for the magnitude of the observed THE.

Efficient Spin-Flip between Charge-Transfer States for High-Performance Electroluminescence, without an Intermediate Locally Excited State.

Thermally activated delayed fluorescence (TADF) materials with both high photoluminescence quantum yield (PLQY) and fast reverse intersystem crossing (RISC) are strongly desired to realize efficient and stable organic light-emitting diodes (OLEDs). Control of excited-state dynamics via molecular design plays a central role in optimizing the PLQY and RISC rate of TADF materials but remains challenging. Here, 3 TADF emitters possessing similar molecular structures, similar high PLQYs (89.5% to 96.3%), and approximate energy levels of the lowest excited singlet states (S1), but significantly different spin-flipping RISC rates (0.03 × 106 s-1 vs. 2.26 × 106 s-1) and exciton lifetime (297.1 to 332.8 μs vs. 6.0 μs) were systematically synthesized to deeply investigate the feasibility of spin-flip between charge-transfer excited states (3CT-1CT) transition. Experimental and theoretical studies reveal that the small singlet-triplet energy gap together with low RISC reorganization energy between the 3CT and 1CT states could provide an efficient RISC through fast spin-flip 3CT-1CT transition, without the participation of an intermediate locally excited state, which has previously been recognized as being necessary for realizing fast RISC. Finally, the OLED based on the champion TADF emitter achieves a maximum external quantum efficiency of 27.1%, a tiny efficiency roll-off of 4.1% at 1,000 cd/m2, and a high luminance of 28,150 cd/m2, which are markedly superior to those of the OLEDs employing the other 2 TADF emitters.

Magnetic anisotropy as an evidence of local magnetic moments in paramagnetic phase of Cr-V alloys

Chromium-rich Cr-V alloys exhibit three phases. The paramagnetic phase (P) above Néel temperature (TN), a transverse polarization spin-density-wave (SDW) phase (AF1) below TN and above the spin–flip temperature (TSF), when the longitudinal polarization SDW phase (AF2) is formed. In the paramagnetic phase, Cr exhibit a Pauli susceptibility with weak temperature dependence. Otherwise, the introduction of small amounts of V in Cr not only decreases TN continuously, but also induces a Curie-Weiss behavior (CW) that we associate with local magnetic moments. This behavior is limited up to 0.67%V and magnetic fields of 15 kOe and was also observed for different Cr alloys. The origin of local magnetic moments has been associated with the establishment of local spin-density waves (LSDW) around V impurities. In this work, we presented an investigation of the effects of local magnetic moments in antiferromagnetic phases in Cr-V alloys. The magnetic susceptibility measurements around the spin–flip transition suggest that the local-SDW induces the strong magnetic anisotropy in the long-range order of the spin-density wave as it is revealed in the thermomagnetic results.

Nickel substitution induced reentrant spin glass behavior in EuGa4

Typically, atomic scale disorder is essential to show glassy behavior and amorphous materials can be the optimum candidates in this regard. Generating local disorder in an ordered material by substitution of another metal is a challenging task. In this paper, we successfully substituted Ni in ${\mathrm{EuGa}}_{4}$ (${\mathrm{EuNi}}_{0.37}{\mathrm{Ga}}_{3.63}$) to induce spin glass (SG) behavior. The compound was characterized by single-crystal x-ray diffraction, dc magnetization, frequency-dependent ac susceptibility, isothermal magnetization at various temperatures, spin relaxation, specific heat, and electric resistivity. Two distinct anomalies were observed in dc susceptibility $\ensuremath{\sim}16.5$ and 5.2 K corresponding to long-range antiferromagnetic (AFM) ordering and SG transition, respectively. Their characteristic peaks in specific heat capacity further support the AFM and SG behavior. The irreversibility temperature is fitted with the Almeida-Thouless equation, confirming the Ising SG. The SG behavior is further confirmed by shift of maxima to a high temperature in ac susceptibility with increasing frequency. Nonzero spin relaxation after 7200 s, fitted with stretched exponential model, provides the value of $\ensuremath{\beta}=0.41$. This material can be identified as a possible reentrant SG as AFM and SG behavior coexist in the low-temperature range. Additionally, a spin-flip transition is observed in the AFM region ($5.2\phantom{\rule{0.28em}{0ex}}\mathrm{K}<T<16.5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$). Our first-principles calculations show Ni substitution induces the distortion in electronic band structure as compared with parent ${\mathrm{EuGa}}_{4}$. A minimal difference between the different magnetic configurations for Ni-substituted ${\mathrm{EuGa}}_{4}$ suggests the frustration in the lattice, pointing toward SG behavior.

Manipulating Exciton Dynamics toward Simultaneous High-Efficiency Narrowband Electroluminescence and Photon Upconversion by a Selenium-Incorporated Multiresonance Delayed Fluorescence Emitter.

Multiresonance thermal activated delayed fluorescence (MR-TADF) materials with an efficient spin-flip transition between singlet and triplet excited states remain demanding. Herein, we report an MR-TADF compound (BN-Se) simultaneously possessing efficient (reverse) intersystem crossing (ISC/RISC), fast radiative decay, close-to-unity quantum yield, and narrowband emission by embedding a single selenium atom into a common 4,4'-diazaborin framework. Benefitting from the high RISC efficiency accelerated by the heavy-atom effect, organic light-emitting diodes (OLEDs) based on BN-Se manifest excellent performance with an external quantum efficiency of up to 32.6% and an ultralow efficiency roll-off of 1.3% at 1000 cd m-2. Furthermore, the high ISC efficiency and small inherent energy loss also render BN-Se a superior photosensitizer to realize the first example of visible (λex > 450 nm)-to-UV (λem < 350 nm) triplet-triplet annihilation upconversion, with a high efficiency (21.4%) and an extremely low threshold intensity (1.3 mW cm-2). This work not only aids in designing advanced pure organic molecules with fast exciton dynamics but also highlights the value of MR-TADF compounds beyond OLED applications.

Anomalous symmetry breaking in the Weyl semimetal CeAlGe

CeAlGe, a proposed type-II Weyl semimetal, orders antiferromagnetically below 5 K. At 2 K, spin-flop and a spin-flip transitions to less than 1 $\mu_B$/Ce are observed in the $M(H)$ data below 30 kOe, ($\bf{H}\|\bf{a}$ and $\bf{b}$, and 4.3 kOe, $\bf{H}\|\langle110\rangle$, respectively, indicating a four-fold symmetry of the $M(H)$ data along the principal directions in the tetragonal $\it{ab}$ plane with $\langle110\rangle$ set of easy directions. However, anomalously robust and complex twofold symmetry is observed in the angular dependence of resistivity and magnetic torque data in the magnetically ordered state once the field is swept in the $\it{ab}$ plane. This twofold symmetry is independent of temperature and field hystereses and suggests a magnetic phase transition that separates two different magnetic structures in the $\it{ab}$ plane. The boundary of this magnetic phase transition and possibly the type of low-field magnetic structure can be tuned by an Al deficiency.

The evolution of structure and magnetic properties of GdCr1-Fe O3 (0 ≤ x ≤ 0.5) polycrystalline samples

In this paper, the evolution of crystal structures and magnetic properties of GdCr 1- x Fe x O 3 (0 ≤ x ≤ 0.5) samples with different doping levels was systematically investigated by powder X-ray diffraction, Fourier transform infrared spectra, positron annihilation spectra, and temperature-dependent and field-dependent magnetization measurements. The lattice constants and Cr(Fe)O 6 octahedral deformation increase with Fe doping showing an anomaly at x = 0.4 which is further confirmed by positron annihilation and Fourier transform spectra. Fe doping in GdCrO 3 can suppress the magnetization reversal in the parent phase as well as its antiferromagnetic ordering temperature T N at low doping levels but enhance T N at high doping levels. A possible spin flip or flop transition under field-cooled-cooling process occurs for the samples of x = 0.3 and 0.4 at low temperature. The coercive fields of weak ferromagnetic state with different doping levels at 10 and 100 K also exhibit a local maximum at around x = 0.4 which is accordingly correlated to the concentrations of the vacancy defects in the sample. • The information of cationic vacancy defects is analyzed by positron annihilation technique. • The correlation between lattice parameters and vacancy defects is unveiled. • Defects in GdCr 1- x Fe x O 3 samples have a great impact on their magnetic properties. • Chemical doping and defect engineering are effective tools to manipulate R CrO 3 materials.

Aligning a Majorana fermion’s anapole moment with an external current through photon emission mediated by the fermion’s generalized polarizabilities

The sole static electromagnetic property of a spin-$\frac{1}{2}$ Majorana fermion is its anapole moment. Though they cannot couple to single real photons, these particles can interact with electric currents through virtual photons. If a Majorana fermion is immersed in a background current, there is an energy difference between the spin states of the fermion; the higher energy state has its anapole moment antialigned with the current. In this paper, we address the ability of a system of initially unpolarized Majorana fermions to achieve some degree of polarization relative to a static background current. In considering processes that allow the Majorana fermion's spin to flip to the lower-energy state, we focus upon two irreversible processes: the spontaneous emission of two real photons and the emission of a single real photon emitted in virtual Compton scattering. Both of these processes involve coupling to photons via the fermion's polarizaibilities. We compute the spin-flip transition rates for these processes using a low-energy expansion of the Hamiltonian and construct a toy model to showcase how these rates depend upon the underlying parameters within a model. Applying these ideas to a thermal dark matter (DM) model, we find that when the DM thermally decouples from the Standard Model plasma in the early universe, two-photon emission is negligible but partial polarization for the DM medium can proceed via virtual Compton scattering if sufficient currents exist.