Nanomechanical absorption spectroscopy of 2D materials with femtowatt sensitivity

Nanomechanical spectroscopy (NMS) is a recently developed approach to determine optical absorption spectra of nanoscale materials via mechanical measurements. It is based on measuring changes in the resonance frequency of a membrane resonator vs. the photon energy of incoming light. This method is a direct measurement of absorption, which has practical advantages compared to common optical spectroscopy approaches. In the case of two-dimensional (2D) materials, NMS overcomes limitations inherent to conventional optical methods, such as the complications associated with measurements at high magnetic fields and low temperatures. In this work, we develop a protocol for NMS of 2D materials that yields two orders of magnitude improved sensitivity compared to previous approaches, while being simpler to use. To this end, we use mechanical sample actuation, which simplifies the experiment and provides a reliable calibration for greater accuracy. Additionally, the use of low-stress silicon nitride membranes as our substrate reduces the noise-equivalent power to $\textrm {NEP} = 890$ fW $\sqrt{\mathrm{Hz}}^{-1}$, comparable to commercial semiconductor photodetectors. We use our approach to spectroscopically characterize a 2D transition metal dichalcogenide (WS2), a layered magnetic semiconductor (CrPS4), and a plasmonic super-crystal consisting of gold nanoparticles

Highly Anisotropic Mechanical Response of the Van der Waals Magnet CrPS4

Semiconducting van der Waals magnets exhibit a rich physical phenomenology with different collective excitations, as magnons or excitons, that can be coupled, thereby offering new opportunities for optoelectronic, spintronic, and magnonic devices. In contrast with the well-studied van der Waals magnets CrI3 or Fe3GeTe2, CrPS4 is a layered metamagnet with a high optical and magnon transport anisotropy. Here, the structural anisotropy of CrPS4 above and below the magnetic phase transition is investigated by fabricating nanomechanical resonators. A large anisotropy is observed in the resonance frequency of resonators oriented along the crystalline a- and b-axis, indicative of a lattice expansion along the b-axis, boosted at the magnetic phase transition, and a rather small continuous contraction along the a-axis. This behavior in the mechanical response differs from that previously reported in van der Waals magnets, as FePS3 or CoPS3, and can be understood from the quasi-1D nature of CrPS4. The results pinpoint CrPS4 as a promising material in the field of low-dimensional magnetism and show the potential of mechanical resonators for unraveling the in-plane structural anisotropy coupled to the magnetic ordering that, in a broader context, can be extended to studying structural modifications in other 2D materials and van der Waals heterostructures

Controlling magnetism with light in zero orbital angular momentum antiferromagnet

Antiferromagnetic materials feature intrinsic ultrafast spin dynamics, making them ideal candidates for future magnonic devices operating at THz frequencies. A major focus of current research is the investigation of optical methods for the efficient generation of coherent magnons in antiferromagnetic insulators. In magnetic lattices endowed with orbital angular momentum, spin-orbit coupling enables spin dynamics through the resonant excitation of low-energy electric dipoles such as phonons and orbital resonances which interact with spins. However, in magnetic systems with zero orbital angular momentum, microscopic pathways for the resonant and low-energy optical excitation of coherent spin dynamics are lacking. Here, we consider experimentally the relative merits of electronic and vibrational excitations for the optical control of zero orbital angular momentum magnets, focusing on a limit case: the antiferromagnet manganese phosphorous trisulfide (MnPS3), constituted by orbital singlet Mn2þ ions. We study the correlation of spins with two types of excitations within its band gap: a bound electron orbital excitation from the singlet orbital ground state of Mn2þ into an orbital triplet state, which causes coherent spin precession, and a vibrational excitation of the crystal field that causes thermal spin disorder. Our findings cast orbital transitions as key targets for magnetic control in insulators constituted by magnetic centers of zero orbital angular momentum

Nanomechanical probing and strain tuning of the Curie temperature in suspended Cr2Ge2Te6-based heterostructures

Two-dimensional magnetic materials with strong magnetostriction are attractive systems for realizing strain-tuning of the magnetization in spintronic and nanomagnetic devices. This requires an understanding of the magneto-mechanical coupling in these materials. In this work, we suspend thin Cr2Ge2Te6 layers and their heterostructures, creating ferromagnetic nanomechanical membrane resonators. We probe their mechanical and magnetic properties as a function of temperature and strain by observing magneto-elastic signatures in the temperature-dependent resonance frequency near the Curie temperature, TC. We compensate for the negative thermal expansion coefficient of Cr2Ge2Te6 by fabricating heterostructures with thin layers of WSe2 and antiferromagnetic FePS3, which have positive thermal expansion coefficients. Thus we demonstrate the possibility of probing multiple magnetic phase transitions in a single heterostructure. Finally, we demonstrate a strain-induced enhancement of TC in a suspended Cr2Ge2Te6-based heterostructure by 2.5 ± 0.6 K by applying a strain of 0.026% via electrostatic force

Study of charge density waves in suspended 2H-TaS2 and 2H-TaSe2 by nanomechanical resonance

The charge density wave (CDW) state in van der Waals systems shows interesting scaling phenomena as the number of layers can significantly affect the CDW transition temperature, TCDW. However, it is often difficult to use conventional methods to study the phase transition in these systems due to their small size and sensitivity to degradation. Degradation is an important parameter, which has been shown to greatly influence the superconductivity in layered systems. Since the CDW state competes with the onset of superconductivity, it is expected that TCDW will also be affected by the degradation. Here, we probe the CDW phase transition by the mechanical resonances of suspended 2H-TaS2 and 2H-TaSe2 membranes and study the effect of disorder on the CDW state. Pristine flakes show the transition near the reported values of 75 K and 122 K, respectively. We then study the effect of degradation on 2H-TaS2, which displays an enhancement of TCDW up to 129 K after degradation in ambient air. Finally, we study a sample with local degradation and observe that multiple phase transitions occur at 87 K, 103 K, and 118 K with a hysteresis in temperature in the same membrane. The observed spatial variations in the Raman spectra suggest that variations in crystal structure cause domains with different transition temperatures, which could result in the hysteresis. This work shows the potential of using nanomechanical resonance to characterize the CDW in suspended 2D materials and demonstrates that the degradation can have a large effect on transition temperatures.the European Union (ERC AdG Mol-2D No. 788222), the Spanish MICINN (No. MAT2017–89993-R and Excellence Unit “María de Maeztu,” No. CEX2019–000919-M), and the Generalitat Valenciana (PO FEDER Program, Nos. IDIFEDER/2018/061 and PROMETEO)

Field dependence of the vortex core size probed by scanning tunneling microscopy

We study the spatial distribution of the density of states (DOS) at zero bias N(r) in the mixed state of single and multigap superconductors. We provide an analytic expression for N(r) based on deGennes' relationship between DOS and the order parameter that reproduces well scanning tunneling microscopy (STM) data in several superconducting materials. In the single gap superconductor β-Bi2Pd, we find that N(r) is governed by a length scale ξH = √φ0/2πH, which decreases in rising fields. The vortex core size C, defined via the slope of the order parameter at the vortex center, C ∝ (d%/dr|r→0)−1, differs from ξH by a material dependent numerical factor. The new data on the tunneling conductance and vortex lattice of the 2H-NbSe1.8S0.2 show the in-plane isotropic vortices, suggesting that substitutional scattering removes the in-plane anisotropy found in the two-gap superconductor 2H-NbSe2. We fit the tunneling conductance of 2H-NbSe1.8S0.2 to a two gap model and calculate the vortex core size C for each band. We find that C is field independent and has the same value for both bands. We also analyze the two-band superconductor 2H-NbS2 and find the same result. We conclude that, independently of the magnetic field induced variation of the order parameter values in both bands, the spatial variation of the order parameter close to the vortex core is the same for all bands

Publisher's Note: “Attosecond state-resolved carrier motion in quantum materials probed by soft x-ray XANES” [Appl. Phys Rev. 8, 011408 (2021)]

Recent developments in attosecond technology led to table-top x-ray spectroscopy in the soft x-ray range, thus uniting the element- and state-specificity of core-level x-ray absorption spectroscopy with the time resolution to follow electronic dynamics in real-time. We describe recent work in attosecond technology and investigations into materials such as Si, SiO2, GaN, Al2O3, Ti, and TiO2, enabled by the convergence of these two capabilities. We showcase the state-of-the-art on isolated attosecond soft x-ray pulses for x-ray absorption near-edge spectroscopy to observe the 3d-state dynamics of the semi-metal TiS2 with attosecond resolution at the Ti L-edge (460 eV). We describe how the element- and state-specificity at the transition metal L-edge of the quantum material allows us to unambiguously identify how and where the optical field influences charge carriers. This precision elucidates that the Ti:3d conduction band states are efficiently photo-doped to a density of 1.9 × 1021 cm−3. The light-field induces coherent motion of intra-band carriers across 38% of the first Brillouin zone. Lastly, we describe the prospects with such unambiguous real-time observation of carrier dynamics in specific bonding or anti-bonding states and speculate that such capability will bring unprecedented opportunities toward an engineered approach for designer materials with pre-defined properties and efficiency. Examples are composites of semiconductors and insulators like Si, Ge, SiO2, GaN, BN, and quantum materials like graphene, transition metal dichalcogens, or high-Tc superconductors like NbN or LaBaCuO. Exiting are prospects to scrutinize canonical questions in multi-body physics, such as whether the electrons or lattice trigger phase transitions

Enhanced superconductivity in atomically thin TaS2

The ability to exfoliate layered materials down to the single layer limit has presented the opportunity to understand how a gradual reduction in dimensionality affects the properties of bulk materials. Here we use this top-down approach to address the problem of superconductivity in the two-dimensional limit. The transport properties of electronic devices based on 2H tantalum disulfide flakes of different thicknesses are presented. We observe that superconductivity persists down to the thinnest layer investigated (3.5 nm), and interestingly, we find a pronounced enhancement in the critical temperature from 0.5 to 2.2 K as the layers are thinned down. In addition, we propose a tight-binding model, which allows us to attribute this phenomenon to an enhancement of the effective electron-phonon coupling constant. This work provides evidence that reducing the dimensionality can strengthen superconductivity as opposed to the weakening effect that has been reported in other 2D materials so far

Isoreticular two-dimensional magnetic coordination polymers prepared through pre-synthetic ligand functionalization

Chemical functionalization is a powerful approach to tailor the physical and chemical properties of two-dimensional materials, increase their processability and stability, tune their functionalities and, even, create new 2D materials. This is typically achieved through post-synthetic functionalization by anchoring molecules on the surface of an exfoliated 2D crystal, but it inevitably alters the long-range structural order of the material. Here we present a pre-synthetic approach that allows the isolation of crystalline, robust, and magnetic functionalized monolayers of coordination polymers. A series of five isoreticular layered magnetic coordination polymers based on Fe(II) centres and different benzimidazole derivatives (bearing a Cl, H, CH3, Br or NH2 side group) were first prepared. On mechanical exfoliation, 2D materials are obtained that retain their long-range structural order and exhibit good mechanical and magnetic properties. This combination, together with the possibility to functionalize their surface at will, makes them good candidates to explore magnetism in the 2D limit and to fabricate mechanical resonators for selective gas sensing