Multichannel direct detection of light dark matter: Target comparison

Probing light inelastic dark matter from direct detection

We investigate the excitation of coherent acoustic and optical phonons by ultrashort extreme ultraviolet (EUV) pulses produced by a free electron laser. Two crossed femtosecond EUV (wavelength 12.7 nm) pulses are used to excite coherent phonons at a wavelength of 280 nm, which are detected via diffraction of an optical probe beam. Longitudinal and surface acoustic waves are measured in BK-7 glass, diamond, and Bi4Ge3O12; in the latter material, the excitation of a coherent optical phonon mode is also observed. We discuss probing different acoustic modes in reflection and transmission geometries and frequency mixing of surface and bulk acoustic waves in the signal. The use of extreme ultraviolet radiation will allow the creation of tunable GHz to THz acoustic sources in any material without the need to fabricate transducer structures.

In a recent paper [Lagos et al., Nature (London) 543, 533 (2017)] we have used electron energy loss spectroscopy with sub-10 meV energy and atomic spatial resolution to map optical and acoustic, bulk and surface vibrational modes in magnesium oxide nanocubes. We found that a local dielectric description works well for the simulation of aloof geometries, similar to related work for surface plasmons and surface plasmon polaritons, while for intersecting geometries such a description fails to reproduce the rich spectral features associated with excitation of bulk acoustic and optical phonons. To account for scatterings with a finite momentum exchange, in this paper we investigate molecular and lattice dynamics simulations of bulk losses in magnesium-oxide nanocubes using a rigid-ion description and investigate the loss spectra for intersecting electron beams. From our analysis we can evaluate the capability of electron energy loss spectroscopy for the investigation of phonon modes at the nanoscale, and we discuss shortcomings of our simplified approach as well as directions for future investigations.

We investigate coherent Bloch oscillations in GaAs/AlxGa1-xAs superlattices with electronic miniband widths larger than the optical phonon energy. In these superlattices the Bloch frequency can be tuned into resonance with the optical phonon. Close to resonance a direct coupling of Bloch oscillations to LO phonons is observed which gives rise to the coherent excitation of LO phonons. The density necessary for driving coherent LO phonons via Bloch oscillations is about 2 orders of magnitude smaller than the density necessary to drive coherent LO phonons in bulk GaAs. The experimental observations are confirmed by the theoretical description of this phenomenon [A.W. Ghosh et al., Phys. Rev. Lett. 85, 1084 (2000)].

Direct detection experiments for dark matter are increasingly ruling out large parameter spaces. However, light dark matter models with particle masses < GeV are still largely unconstrained. Here we examine a proposal to use atom interferometers to detect a light dark matter subcomponent at sub-GeV masses. We describe the decoherence and phase shifts caused by dark matter scattering off of one “arm” of an atom interferometer using a generalized dark matter direct detection framework. This allows us to consider multiple channels: nuclear recoils, hidden photon processes, and axion interactions. We apply this framework to several proposed atom interferometer experiments. Because atom interferometers are sensitive to extremely low momentum deposition and their coherent atoms give them a boost in sensitivity, these experiments will be highly competitive and complementary to other direct detection methods. In particular, atom interferometers are uniquely able to probe a dark matter subcomponent with m<sub>χ</sub> ≲ 10 keV. We find that, for a mediator mass m<sub>φ</sub> = 10<sup>-10</sup>m<sub>χ</sub>, future atom interferometers could close a gap in the existing constraints on nuclear recoils down to σ¯<sub>n</sub> ~ 10<sup>-50</sup> cm<sup>2</sup> for m<sub>χ</sub> ~ 10<sup>-5</sup>–10<sup>-1</sup> MeV dark matter masses.

The mass of the dark matter particle is unknown, and may be as low as ~$10^{-22}$ eV. The lighter part of this range, below ~eV, is relatively unexplored both theoretically and experimentally but contains an array of natural dark matter candidates. An example is the relaxion, a light boson predicted by cosmological solutions to the hierarchy problem. One of the few generic signals such light dark matter can produce is a time-oscillating, EP-violating force. We propose searches for this using accelerometers, and consider in detail the examples of torsion balances, atom interferometry, and pulsar timing. These approaches have the potential to probe large parts of unexplored parameter space in the next several years. Thus such accelerometers provide radically new avenues for the direct detection of dark matter.

The discovery oftwo-dimensional (2D) magnetic materials that haveexcellent piezoelectric response is promising for nanoscale multifunctionalpiezoelectric or spintronic devices. Piezoelectricity requires a noncentrosymmetricstructure with an electronic band gap, whereas magnetism demands brokentime-reversal symmetry. Most of the well-known 2D piezoelectrics,e.g., 1H-MoS2 monolayer, are not magnetic. Being intrinsicallymagnetic, semiconducting 1H-LaBr2 and 1H-VS2 monolayers can combine magnetism and piezoelectricity. We comparepiezoelectric properties of 1H-MoS2, 1H-VS2,and 1H-LaBr2 using density functional theory. The ferromagnetic1H-LaBr2 and 1H-VS2 monolayers display largerpiezoelectric strain coefficients, namely, d11 = −4.527 pm/V for 1H-LaBr2 and d11 = 4.104 pm/V for 1H-VS2, comparedto 1H-MoS2 (d11 = 3.706 pm/V).1H-MoS2 has a larger piezoelectric stress coefficient (e11 = 370.675 pC/m) than 1H-LaBr2 (e11 = −94.175 pC/m) and 1H-VS2 (e11 = 298.100 pC/m). The large d11 for 1H-LaBr2 originates from thelow elastic constants, C11 = 30.338 N/mand C12 = 9.534 N/m. The sign of the piezoelectriccoefficients for 1H-LaBr2 is negative, and this arisesfrom the negative ionic contribution of e11, which dominates in 1H-LaBr2, whereas the electronicpart of e11 dominates in 1H-MoS2 and 1H-VS2. We explain the origin of this large ioniccontribution of e11 for 1H-LaBr2 through Born effective charges (Z11)and the sensitivity of the atomic positions to the strain (du/dη). We observe a sign reversal in the Z11 values of Mo and S compared to the nominal oxidationstates, which makes both the electronic and ionic parts of e11 positive and results in the high value of e11. We also show that a change in magnetic ordercan enhance (reduce) the piezoresponse of 1H-LaBr2 (1H-VS2).

Detections of non-gravitational interactions of massive dark matter (DM) with visible sector so far have given null results. The DM may communicate with the ordinary matter only through gravitational interaction. Besides, the majority of traditional direct detections have poor sensitivities for light DM because of the small recoil energy. Thanks to the high energy cosmic rays (CRs), the light DM can be boosted by scattering with CRs and thus may be detected in the ongoing experiments. In this work, we derive the exclusion limits on the cosmic ray boosted sub-GeV DM with gravitational mediator from the Xenon1T data. It turns out that a sizable region of such a cosmic ray boosted DM can be excluded by the current data.

Data acquisition electronics and reconstruction software for directional detection of dark matter with MIMAC

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Electronic energy state, optical phonon, and photo excitation in high ionic conductors

Electron-phonon superconductors at high pressures have displayed the highest values of critical superconducting temperature ${T}_{c}$ on record, now rapidly approaching room temperature. Despite the importance of high-$P$ superconductivity in the quest for room-temperature superconductors, a mechanistic understanding of the effect of pressure and its complex interplay with phonon anharmonicity and superconductivity is missing, as numerical simulations can bring only system-specific details, clouding out key players controlling the physics. Here we develop a minimal model of electron-phonon superconductivity under an applied pressure which takes into account the anharmonic decoherence of the optical phonons. We find that ${T}_{c}$ behaves nonmonotonically as a function of the ratio $\mathrm{\ensuremath{\Gamma}}/{\ensuremath{\omega}}_{0}$, where $\mathrm{\ensuremath{\Gamma}}$ is the optical phonon damping and ${\ensuremath{\omega}}_{0}$ is the optical phonon energy at zero pressure and momentum. Optimal pairing occurs for a critical ratio $\mathrm{\ensuremath{\Gamma}}/{\ensuremath{\omega}}_{0}$ when the phonons are on the verge of decoherence (``diffusonlike'' limit). Our framework gives insights into recent experimental observations of ${T}_{c}$ as a function of pressure in the complex BCS material ${\mathrm{TlInTe}}_{2}$.

Intersubbands carrier dynamics in quantum wire structures

Polar materials with optical phonons in the meV range are excellent candidates for both dark matter direct detection via dark photon-mediated scattering and light dark matter absorption. In this study, we propose, for the first time, the metal halide perovskites MAPbI 3 , MAPbCl 3 , and CsPbI 3 for these purposes. Our findings reveal that CsPbI 3 is the best material, significantly improving exclusion limits compared to other polar materials. For scattering, CsPbI 3 can probe dark matter masses down to the keV range. For absorption, it enhances sensitivity to detect dark photon masses below ∼ 10 meV . The only material that has so far been investigated and that could provide competitive bounds is CsI, which, however, demonstrates lower stability as device platform compared to CsPbI 3 . Moreover, CsI is isotropic while the anisotropic structure of CsPbI 3 enables daily modulation analysis, showing that a significant percentage of daily modulation exceeding 1% is achievable for dark matter masses below 40 keV.

The interaction of cesium atoms with graphitized 4H-SiC(0001) surfaces was investigated by use of Auger electron, ultraviolet photoemission and high-resolution electron energy-loss spectroscopy (HREELS) as well as work function measurements with a Kelvin probe. The samples were annealed at temperatures well above 1300 K resulting in a formation of about two graphite layers on top of the SiC substrates. The corresponding low-energy electron diffraction image shows a () pattern. Cs deposition on graphitized SiC occurs in two subsequent stages. First, even small amounts of evaporated Cs lead to a spontaneous decrease in work function by 1.8 eV. The work function stays constant after further evaporation corresponding to an intercalation of Cs into the graphite overlayer. In the second stage Cs atoms are adsorbed on top of the Cs-intercalated compound resulting in a further decrease of work function. The graphite layer is effectively metallized by Cs evaporation. This is confirmed by a Fermi edge which is observed in the photoemission spectra even after the smallest dose applied. Moreover, the excitation of optical Fuchs-Kliewer phonons in SiC by HREELS is significantly quenched by the metallic overlayer and the electron energy-loss spectra exhibit features which are typical for thin homogeneous metal layers on surfaces of heteropolar semiconductors.